When President Joseph R. Biden Jr. convened democracy summits at the White House in 2021 and 2023, he pointedly disinvited President Recep Tayyip Erdogan of Turkey, a man he had once described an “autocrat” who deserved to be driven from office by voters.

On Tuesday, President Trump offered a much rosier assessment of the Turkish president, even as protesters filled the streets following the arrest of the mayor of Istanbul, Mr. Erdogan’s chief political rival.

“A good leader,” the president said of Mr. Erdogan during a meeting of his ambassadors at the White House. He made no mention of the arrest or the protests.

Since taking office 66 days ago, Mr. Trump has turned a central precept of American diplomacy on its head. He is embracing — rather than denouncing — fellow leaders who abandon democratic principles. The longstanding bipartisan effort to bolster democratic institutions around the globe has been replaced by a president who praises leaders who move toward autocracy.

And Mr. Trump’s own actions — taking revenge against his political rivals, attacking law firms, journalists and universities, and questioning the authority of the judiciary — are offering new models for democratically elected leaders in countries like Serbia and Israel who have already shown their willingness to push the boundaries of their own institutions.

“There’s a great emboldening,” said Rosa Balfour, the Europe director for the Carnegie Endowment for International Peace. “What Trump says reverberates strongly here. But also what the United States does not do. It does not punish or condemn any attempt to undermine rule of law or democracy. There are no repercussions.”

Jane Harman, a former member of Congress and former president of the Woodrow Wilson International Center for Scholars, noted that Mr. Erdogan and other leaders around the world had been “drifting away” from democratic principles for years.

In 2016, a faction in Mr. Erdogan’s government attempted a coup to overthrow him. Since then, he has tightened control of the presidency by attacking the media, political opponents, the courts and other institutions.

“This has become a very different world, but I don’t think Trump started it, and I don’t think Trump is going to end it either,” Ms. Harman said. And she noted that in at least a few places, Mr. Trump’s return to power had prompted some voters to question the authoritarian leanings of candidates and parties.

“Think Germany,” she said, referring to recent elections in the country. “The far right has risen in popularity, but it didn’t win. And the backlash to Trump might have been part of the momentum that held it back.”

Mr. Trump is not the first president to tolerate less-than-democratic actions from allies when they deemed it necessary.

Mr. Biden offered a fist-bump to Crown Prince Mohammed bin Salman, Saudi Arabia’s de facto ruler, even as he blamed him for the murder of the columnist Jamal Khashoggi. Mr. Biden also worked with Prime Minister Narendra Modi of India, who has increasingly cracked down on dissent in his country, and — at times — with Mr. Erdogan.

But Mr. Trump’s election has coincided with actions by elected leaders that appear to depart from the kind of democratic principles that America stood for.

In Israel, Prime Minister Benjamin Netanyahu no longer needs to contend with Mr. Biden’s opposition to a long-planned overhaul of the courts, which many Israelis view as an attempt to control and politicize the judiciary. In 2023, Mr. Biden told reporters that Mr. Netanyahu “cannot continue down this road” of judicial changes.

Now, with Mr. Trump in office, the Israeli leader faces no such pressure. This month, he fired the chief of the country’s domestic intelligence agency, a move seen as undermining its independence. Later, the cabinet approved a vote of no confidence in the country’s attorney general, prompting fresh accusations that Mr. Netanyahu is curbing the independence of the justice system, purging officials he considers disloyal.

On Thursday, Mr. Netanyahu’s allies in Parliament voted to give themselves more power over the selection of the country’s judges. The vote came after the prime minister gave a speech echoing Mr. Trump and saying that the action meant that “the deep state is in danger.”

“The U.S. is not going to put any pressure whatsoever on Netanyahu to respect the democratic institutions of his own country,” Ms. Balfour said. “Netanyahu feels that he has impunity in that respect.”

In Serbia, President Aleksandar Vucic has spent years attacking the media and other political opponents. Last month — as Mr. Trump dismantled the U.S. Agency for International Development — Mr. Vucic sent police to raid organizations in his country, some of which had received money from the now largely shuttered American agency.

Authorities in Mr. Vucic’s government cited Mr. Trump’s actions in the United States as justification for moving against the organizations, including the Centre for Research, Transparency and Accountability and Civic Initiatives. They quoted Elon Musk, the multibillionaire who is running the so-called Department of Government Efficiency, who claimed, without evidence, that USAID was a “criminal organization.”

Two weeks after the raids in Serbia, Donald Trump Jr., the president’s eldest son, traveled to Belgrade, the country’s capital, to interview Mr. Vucic for his podcast. In the interview, Mr. Vucic complained that he, like the American president, is opposed by “an entire liberal establishment from Washington and New York and L.A. going against you.” He said the raids of the nongovernmental organizations were designed to root out corruption and financial mismanagement.

Mr. Trump Jr. fawned over Mr. Vucic, describing what he called “an embrace of common sense, an embrace of law and order, of a shared national sense of identity.” He criticized protesters angry about Mr. Vucic’s recent actions.

“I’m sure the media will cover them only one way,” Mr. Trump Jr. said. “And now there’s seemingly evidence that they are all tied in some form to the same left-wing actors here in America. That same propaganda machine.”

The president’s son is not the only one echoing his father’s language.

Last week, after Mr. Erdogan’s government jailed the mayor of Istanbul, one of Mr. Trump’s senior diplomatic envoys spoke positively about Turkey’s leader during an interview with the former Fox News host Tucker Carlson.

“Really transformational,” Steve Witkoff said of a recent telephone call between Mr. Trump and Mr. Erdogan. “There’s just a lot of good, positive news coming out of Turkey right now as a result of that conversation.”

Ruth Ben-Ghiat, a professor of history at New York University, said Mr. Trump’s words and actions — and those of his surrogates — are being watched by other leaders. She said the president’s lack of condemnation of Mr. Erdogan following the arrest of the Istanbul mayor would have been noted by authoritarian-leaning presidents and prime ministers.

“The moves of Trump in this same direction,” she said, “embolden foreign leaders who know the U.S. is now an autocratic ally and there will be no consequences for repressive behavior.”