Kash Patel spent years ingratiating himself with Donald J. Trump — regularly popping into the Oval Office in the first term, writing a children’s book starring “King Donald” during the interregnum, trailing him to rallies, banquets and bus tours on the bumpy ride back to power.

Few practitioners of the audience-of-one strategy have been quite so successful at translating loyalty and proximity to Mr. Trump into real influence. Fewer still are poised to be rewarded as significantly as Mr. Patel, 44, Mr. Trump’s pick to run the F.B.I., an agency with vast powers that he has vowed to radically overhaul.

What binds Mr. Trump and Mr. Patel is the shared conviction that the bureau has been weaponized against conservatives, including both of them. They argue it is politicized and the only way to fix it is to empower an outsider willing to faithfully execute the Trump agenda — a sharp divergence from the bureau’s historical norms and the decades-long practice of directors’ limiting contact with presidents.

The issue of Mr. Patel’s independence, or lack thereof, will be a flashpoint at a confirmation hearing scheduled for Thursday.

Mr. Patel’s embrace of Jan. 6 conspiracy theories and unflinching fealty are the coin of the realm in Mr. Trump’s orbit. But in the view of his many critics (and even some who publicly sing his praises), Mr. Patel’s oft-stated loyalty to the president poses one of the most significant challenges to the independence of the F.B.I. in the century since J. Edgar Hoover, its founding director, built an investigative citadel whose autonomy created leverage, and abuses of power.

Nominating Mr. Patel as F.B.I. chief is, above all, a defining example of Mr. Trump’s approach to exerting power in his second term. Not content to simply install subordinates to help enact an ideological agenda, the president is pushing hard to expand the post-Watergate limits on presidential authority. During his first term, demanding personal loyalty from appointees did not always work; making sure the top jobs are stocked with loyalists is the strategy now.

At the F.B.I., this entails bucking the bureau’s long institutional history, starting with Mr. Hoover and extending through James B. Comey’s rejection of Mr. Trump’s first-term demands for obeisance, a stance that prevented it from becoming the instrument of presidential whim.

Critics say Mr. Trump’s and Mr. Patel’s grievance that the bureau has been “politicized” against Republicans is an excuse to turn the F.B.I., whose agents have often tilted right, into a political weapon for Mr. Trump.

“Hoover would have been appalled at Patel’s sycophancy of Donald Trump,” said Beverly Gage, a professor at Yale and the author of a biography of Mr. Hoover.

“What’s new and alarming about Patel?” she added. “He’s so close to Donald Trump and is making no secret that he will use the bureau to punish Mr. Trump’s enemies. He’s coming in openly hostile to the institution. At the F.B.I., this is potentially earth-shattering.”

The president and Mr. Patel share not only a worldview, but also an enemies list. In 2022, Mr. Patel published a roster of 60 people he suggested should be investigated, prosecuted or otherwise reviled. It includes Christopher A. Wray, who stepped down this month as F.B.I. director before Mr. Trump could fire him, former Attorney General Merrick B. Garland, former Attorney General William P. Barr and a host of other federal officials and politicians he does not like.

Mr. Patel’s spokesman did not respond to questions.

But his defenders downplay his promises to rain hell as campaign-season fireworks, and say the list he published in his book “Government Gangsters” was just a litany of people he did not like, respect or trust. Behind closed doors, he has sought to reassure senators he intended only to underscore the need to reform the bureau and will run it responsibly if confirmed, according to people briefed on the interactions.

In at least one conversation, he has acknowledged that he amped up the verbiage in his polemical memoir for dramatic effect. In another, he apologized for the book, which served as a pugilistic takedown of government officials at the very institution he is eager to run.

“Like me, Kash Patel uses fiery rhetoric and hyperbole to break through,” said Mike Davis, a former Senate Republican staff member who is close to Mr. Patel. “But don’t let that fool anyone. Kash is a very serious, skilled and effective national security operator.”

The team overseeing Mr. Patel’s confirmation has emphasized his unique experience, particularly his work as a public defender, and varied assignments in national security posts.

Yet some Republicans in the Senate have quietly made it clear they want Mr. Trump to surround Mr. Patel with more conventional officials to offset his shortcomings.

Mr. Patel has given private assurances that his deputy director will be a special agent, with deep experience at the bureau, and not a political appointee, according to a person familiar with the matter.

At least two former F.B.I. veterans have been tapped to advise Mr. Patel, including one who recently served as a staff aide to Representative Jim Jordan. While he is seen as a stabilizing force, his past work for Mr. Jordan’s committee uncovering the so-called weaponization of government is in line with Mr. Patel’s worldview.

Mr. Trump is not likely to abide by norms adopted over the past half-century intended to prevent direct interference into federal law enforcement, regardless of who is on staff.

Case in point: The director Mr. Trump signaled he would replace, Mr. Wray, never met alone with Mr. Trump, according to people familiar with the situation. That did not stop Mr. Trump from trying to contact him anyway, at the exact moment the bureau was embarking on its investigation into his retention of national security documents.

In a handwritten note dated March 26, 2022, Mr. Trump congratulated Mr. Wray, whom he appointed in 2017, for an appearance on “60 Minutes,” according to a copy viewed by The New York Times.

“CHRIS – GREAT JOB ON 60 MINUTES LAST NIGHT. YOU ARE 100 % CORRECT ON CHINA (RUSSIA IS NOT SO WONDERFUL EITHER!).”

Mr. Trump does not need to use stationery to reach Mr. Patel.

As a senior director at the National Security Council during Mr. Trump’s first term, Mr. Patel seemed to always find himself invited to the Oval Office for meetings. He also had a knack for trolling Mr. Trump’s enemies — threatening, among other things, to sue the news media for unflattering stories. The president, over time, began to reach out to him for advice.

One former Trump administration official recalled that during the first term, Mr. Patel would head to lunch only to be interrupted by calls from the president to kibitz.

Mr. Patel loved it, the person recalled.

A Break With the Past

The F.B.I. has had a checkered relationship with politics that precedes Mr. Patel by 101 years.

The official origins of the F.B.I. date back to 1908, but its true inception came in 1924 when Mr. Hoover, then in his late 20s, was appointed director. From the start, its mission placed it at the hazardous intersection of politics and law enforcement: investigating, prosecuting and deporting left-wing radicals and anarchists after World War I.

Over the decades, Mr. Hoover leveraged his cache of investigative files into raw power. Toward the end of his 48-year tenure, he greenlit dozens of investigations of key figures in the civil rights movement — most infamously Martin Luther King Jr. — and offered political intelligence to presidents and their political adversaries.

Even while presiding over the bureau’s worst excesses, however, Mr. Hoover ensured that the agency remained independent from direct White House control. Directors who served after him sought to maintain that independence by keeping presidents at arm’s length, with the exception of his immediate successor, L. Patrick Gray.

“Integrity and independence make or break an F.B.I. director,” Louis J. Freeh, the bureau director whose relationship with President Bill Clinton turned rancid as he investigated the president and his associates, said in his memoir.

Mr. Clinton groused but did not seek to remove Mr. Freeh. Mr. Trump did both. In private meetings at the White House, Mr. Trump demanded the loyalty of Mr. Comey, a Republican, and suggested he end an investigation into the president’s former national security adviser. Mr. Comey stayed in office for nearly four months without giving it.

Mr. Comey was confident he could undertake investigations into top public figures, including Mr. Trump and Hillary Clinton, while defending the bureau’s integrity. That miscalculation led to a disastrous news conference in July 2016 at which he announced that although Mrs. Clinton had been “extremely careless” in handling classified information, she would not be prosecuted. Many Democrats believe the assertion ultimately contributed to her defeat.

His approach left the F.B.I. reeling, and Mr. Patel and many other Republicans cite Mr. Comey as one of the main reasons the bureau needs to be reshaped and more agents from its headquarters in Washington farmed out to field offices around the country.

Mr. Wray, who was appointed by Mr. Trump in mid-2017 for a 10-year term, took a much more cautious, conventional approach to Mr. Trump. Nonetheless, their relationship soured almost immediately.

Mr. Trump came close to firing Mr. Wray after he refused, among other things, to embrace the president’s lies about the 2020 election being stolen.

Agents who worked for Mr. Wray described him as fundamentally apolitical, focused on the threat posed by China and other foreign adversaries, and fixated on the minutiae of law enforcement — spending time in briefings on firearms testing, audits of secret surveillance warrants and information technology systems. One former F.B.I. official likened the meetings to watching paint dry, yet the director loved them.

But he could not escape politics. And his commitment to investigating Mr. Trump, including the execution of a search warrant at Mar-a-Lago, effectively doomed his directorship.

On a gray, snow-flecked day at the F.B.I.’s headquarters this month, national security leaders from the United States and Britain gathered to thank Mr. Wray, and to issue barely veiled warnings about what the future might hold if Mr. Trump succeeds in asserting control.

Former top F.B.I. officials were in attendance, including William H. Webster, who was appointed by President Jimmy Carter.

So was William J. Burns, the C.I.A. director, who said Mr. Wray’s greatest achievement was fulfilling a promise he made at his 2017 confirmation hearing to adhere to the “impartial pursuit of justice.”

When it came time for Mr. Wray to speak, he exhorted agents to stay and conduct their investigations with impartiality.

“That means following the facts wherever they lead, no matter who likes it, or doesn’t,” Mr. Wray said. “Because there’s always someone who doesn’t like it.”

A Precipitous Rise

Mr. Patel’s swift ascent in Mr. Trump’s orbit began in 2018. Then a little-known House Republican aide, Mr. Patel investigated the Justice Department’s efforts to obtain a secret surveillance warrant for a Trump adviser believed to be conspiring with the Russians during the 2016 campaign.

From there, he landed a succession of national security posts in rapid succession, serving his longest stint on the National Security Council (20 months) and the shortest as a top aide at the Pentagon (three months). He often communicated with the president directly, to the chagrin of his nominal superiors.

By the spring of 2020, Mr. Trump was eager to dismiss Mr. Wray, replace him with a senior intelligence official and install Mr. Patel as his top deputy in charge, a post typically reserved for a senior agent in a work force of 38,000.

Mr. Barr, then the attorney general, talked Mr. Trump down during a contentious meeting in the Oval Office. Mr. Barr would later write in his memoir that Mr. Patel was deeply unqualified and that the president “showed a shocking detachment from reality.”

People close to Mr. Barr said he was also concerned that Mr. Patel would have been too compliant to challenge Mr. Trump.

Early on, Mr. Wray concluded that limiting contact with the White House, or communicating through intermediaries, could ensure independence, a policy he maintained with Mr. Trump and President Joseph R. Biden Jr.

After Mr. Trump left office, he tapped Mr. Patel as one of his emissaries to the National Archives, thrusting Mr. Patel into the Trump classified documents investigation.

In August 2022, F.B.I. agents and federal prosecutors obtained a court-authorized warrant to search Mr. Trump’s Florida club and residence, including his bedroom. In his book, Mr. Patel said that the “Mar-a-Lago raid will go down in history as a sign of the destruction of our once great institutions of equal justice and fairness.”

During Mr. Trump’s time out of office, Mr. Patel cultivated relationships with the president’s sons, particularly Donald Trump Jr., and embraced online retail (under the brand “K$H”). He also hawked anti-vaccine diet supplements, pro-Trump T-shirts and a line of children’s books in which he portrayed himself as a wizard, wearing a midnight blue robe. Mr. Trump was depicted with a crown.

Mr. Patel, who is single, likes the nightlife. He was recently spotted posing for poolside photos with bikini-clad conservatives, and his Senate disclosure form revealed that he recently joined the Poodle Room, a members-only club near his residence in Las Vegas that has a $20,000 entry fee.

More than anything, he worked relentlessly to raise his profile in Trump circles, doing nearly 1,000 interviews and podcasts. On his Senate disclosure form, he said he “served as a surrogate” for Mr. Trump’s campaign from November 2022 to November 2024.

Mr. Trump has always been leery of subordinates who market themselves off their association with him. And his support of Mr. Patel has been somewhat tempered by doubts about his gravitas and experience. Mr. Trump’s 2024 campaign manager and the new White House chief of staff, Susie Wiles, told him the selection was too risky, associates of both men said.

But the only serious alternative to Mr. Patel that emerged, Missouri’s Republican attorney general, Andrew Bailey, seemed too laid-back and lackluster in face-to-face meetings.

Mr. Patel, always loyal — and always around — lobbied furiously for the job, and prevailed.

After his selection, Mr. Patel appeared to become more cognizant of his attack-dog reputation. Off camera he was more muted, self-effacing, funny and willing to compromise, which allayed the concerns of Ms. Wiles and other skeptics.

Moreover — despite Mr. Patel’s inflammatory public statements — his vetting did not reveal a knockout scandal comparable to the one that forced out Matt Gaetz, Mr. Trump’s initial pick for attorney general.

Mr. Trump did not consult with senators in his own party before nominating Mr. Patel, according to one senator and several aides. Nor did he apparently seek approval from Pam Bondi, his more conventional second choice for attorney general, according to people in his orbit.

The response to Mr. Patel’s appointment among Senate Republicans has been mixed, with some issuing emphatic endorsements and others taking a wait-and-see tack. To allay some concerns, former Representative Trey Gowdy, a former federal prosecutor from South Carolina who is friendly with Mr. Patel, has been furiously working the phones on his behalf, according to people familiar with the situation.

As he has so often done with top aides, Mr. Trump, a former reality TV star, fretted that Mr. Patel lacked the central-casting look the public had come to expect from an F.B.I. director, without either the imposing G-man appearance of a former director like Robert S. Mueller III or the bulldog mien of the bureau’s founder.

“He’s no J. Edgar Hoover,” Mr. Trump told an adviser.

Devlin Barrett and Jonathan Swan contributed reporting.