The trip down that escalator took less than 30 seconds, but it opened a much longer journey for the man and his country.

It has been 10 years now, as of Monday, since Donald J. Trump descended to the lobby of his namesake tower to announce his campaign for president. Ten years of jaw-dropping, woke-busting, scandal-defying, status quo-smashing politics that have transformed America for good or ill in profoundly fundamental ways.

In those 10 years, Mr. Trump has come to define his age in a way rarely seen in America, more so than any president of the past century other than Franklin D. Roosevelt and Ronald Reagan, even though he has never had anywhere near their broad public support. Somehow the most unpopular president in the history of polling has translated the backing of a minority of Americans into the most consequential political force of modern times, rewriting all of the rules along the way.

In a sense, it does not matter that Mr. Trump has actually occupied the White House for less than half of that 10 years. He has shaped and influenced the national discourse since June 16, 2015, whether in office or not. Every issue, every dispute, every conversation on the national level in that time, it seems, has revolved around him.

Even voter repudiation and criminal conviction did not slow him down or diminish his hold on the national imagination on the way to his comeback last November. The presidency of Joseph R. Biden Jr. turned out to be just an interregnum between Mr. Trump’s stints in power.

And power has become his leitmotif. Since his often-stumbling first term, when he was the only president never to have served in public office or the military and by his own admission did not really know what he was doing, Mr. Trump has learned how to wield power to great effect. He has claimed more of it than any of his predecessors ever did — and more of it, judging by the plethora of court rulings against him, than the Constitution entitles him to.

Whether he is on the cusp of dictatorship as his “No Kings” critics argue, he has certainly tried to dictate the course of society across the board, seeking to impose his will not just on Washington but on academia, culture, sports, the legal industry, the news media, Wall Street, Hollywood and private businesses. He wants to personally determine traffic congestion rules in New York and the playbill at the John F. Kennedy Center for the Performing Arts.

And not through the art of persuasion or even the art of the deal, but through the force of threats and intimidation. He has embarked on a campaign of what he has called “retribution” against his political enemies. American troops have been deployed to the streets of Los Angeles to quell protests. Masked agents sweep through towns and cities across the country seizing immigrants, not just the criminals or the undocumented, but in some cases those with all the right papers who in one way or the other offended the president’s sensibilities.

“President Trump has been the dominant figure in American politics since he rode down the escalator to announce his candidacy in 2015,” said Douglas B. Sosnik, a longtime Democratic strategist who served as White House senior adviser to President Bill Clinton. “History will look back and say that we have been living in the age of Trump since then. Biden’s presidency was just a speed bump during this historic period of change in our country’s history.”

It is change that his allies consider a long-overdue course correction after decades of liberal hegemony that they say sought to control not just what Americans did but what they thought and were allowed to say out loud. He is in their view the desperately needed antidote to woke excesses, unrestrained immigration and economic dislocation. His Make America Great Again theme appeals to those who feel left behind and browbeaten by a self-dealing ruling class.

And he has succeeded to an extent that might not have been expected even a few months ago at shaking up the very foundations of the American system as it has been operating for generations, a system he and his allies argue was badly in need of shaking up.

Since reclaiming the presidency five months ago, he has dismantled whole government agencies, overturned the international trading system, gutted federally funded scientific research and made the very word “diversity” so radioactive that even companies and institutions outside his direct control are rushing to change their policies.

Newt Gingrich, the former House speaker who has written multiple books extolling Mr. Trump, said the president had ushered in “a dramatic deep rebellion against a corrupt, increasingly radical establishment breaking the law to stay in power.”

Larry Kudlow, a national economics adviser to Mr. Trump in his first term, said the president “has transformed American thinking on border security, China trade, working-class wage protection and business prosperity.” Moreover, Mr. Kudlow added, “he has fostered a new conservative culture of patriotism, traditional family values, a revival of faith and American greatness.”

But while Mr. Trump’s supporters feel freed from the shackles of a suffocating left-wing elite obsessed with identity politics, his critics see a permission structure for racism, sexism, homophobia, xenophobia, Christian nationalism, white supremacy and hatred of transgender people.

“He sells the past,” said Christina M. Greer, the author of “How to Build a Democracy” and a political science professor at Fordham University. “He sells a version and vision of America that was only accessible to some.” She said that Mr. Trump “has exposed America — a fragile nation that can be torn apart quite quickly by the promise of cruelty. He is returning the nation to its true origin story, one that many would prefer to forget.”

The political shift embodied by Mr. Trump has defied resistance. Brief surges of progressive momentum have faded in the Age of Trump. Despite the #MeToo movement that transformed the American workplace, or perhaps in backlash to it, voters last year elected a president who had been found liable by a jury of sexually abusing a woman. Five years after the widespread protests against the police murder of George Floyd, Mr. Trump pressured Washington’s mayor to erase the “Black Lives Matter” street mural within sight of the White House.

Along the way, he has normalized the abnormal. Not only is he the first convicted felon elected president, but he has also dispensed with conflict-of-interest concerns that used to constrain other presidents and monetized the White House far more than anyone who has ever lived there. He has disproved the assumption that scandal is automatically a political death knell, so much so that even former Gov. Andrew M. Cuomo of New York, who resigned in disgrace amid sexual harassment allegations, is now a front-runner to win election as mayor of New York.

He has also upended the old conventional wisdom that optimism was the key to success in presidential politics. Unlike Roosevelt and Reagan, who projected sunny confidence and offered an idealized view of America, Mr. Trump describes the country in dystopian terms like “hellhole,” “cesspools” and “garbage can.” He is a voice not so much of American greatness as American grievance, one that resonates with many voters.

Politics in the Age of Trump are not kinder and gentler, as President George H.W. Bush once promised, but coarse and corrosive. Mr. Trump seems to love nothing more than an enemy he can insult in scathing, sometimes scatological terms that would be familiar on a school playground but banned on prime-time television.

Others have taken their cue from him. Democrats were thrilled to have Gov. Gavin Newsom of California respond to Mr. Trump’s deployment of the National Guard in tough-guy, bring-it-on terms, daring the president to have him arrested and likening his tactics to those of “failed dictators.”

If the rhetoric is raw, politics have also grown increasingly violent, evoking the darker days of the 1960s. The assassination over the weekend of a Democratic lawmaker from Minnesota and her husband along with the shooting of another legislator and his wife served as a chilling reminder that political discourse has descended to physical danger. So too did two assassination attempts against Mr. Trump during last year’s campaign, one of which came just a couple of inches away from grisly success.

Both Roosevelt and Reagan emerged from assassination attempts with broad public support and sympathy, but today’s America is so polarized that the country is not brought together in moments of crisis for long. Mr. Trump pardoned supporters who beat police officers while storming the Capitol on Jan. 6, 2021, and briefly considered pardoning men who were convicted of plotting to kidnap Gov. Gretchen Whitmer of Michigan, a Democrat, before dropping the idea.

Mr. Trump has brought the political fringe into the mainstream and even the corridors of power, installing people in positions of authority who would never have passed muster in previous administrations. And he has elevated conspiracy theories to the Oval Office, suggesting that the gold might have been stolen from Fort Knox, fanning old suspicions about President John F. Kennedy’s assassination and continuing to lie about his own election defeat in 2020.

For all of the controversies, for all of the conflicts, Mr. Trump maintains a strong hold on his base if not with the broader public. The latest Gallup poll found his approval rating at 43 percent, lower than any other modern president at this stage of a new term or a second term, but essentially right in the same range it has been for most of the time since Mr. Trump stepped off that escalator in 2015.

The uncertainty about the Age of Trump is whether it survives Mr. Trump himself. Will Vice President JD Vance, Secretary of State Marco Rubio, the former Fox News host Tucker Carlson or other aspirants to the throne extend this era beyond its progenitor? Mr. Trump has scrambled old voting blocs and ideological scripts, but will he forge an enduring political and governing coalition?

His appeal often seems as personal as it is political, as much about the force of his identity as the force of his ideas. For a decade, he has been a singularly commanding presence in the life of the nation, invigorating to his admirers and infuriating to his detractors.

The Age of Trump still has more than three and a half years to go, at least by the Constitution, and many tests ahead.