Biden, on a global stage, has harsh words on Trump.

BRUSSELS — President Biden lashed out Monday at what he called former President Donald J. Trump’s “phony populism,” using the global platform of his first NATO summit to criticize his predecessor. But he expressed optimism that the Republican party is beginning to reject the political dominance Mr. Trump has exerted for the past four years.

“I think this is passing,” Mr. Biden told reporters after being asked about the reaction from foreign leaders to the Republican embrace of Mr. Trump’s election falsehoods. “I don’t mean easily passing. That’s why it’s so important that I succeed in my agenda.”

Mr. Biden’s willingness to directly call out his predecessor is a departure from his usual tendency to ignore Mr. Trump. And it came during Mr. Biden’s first foreign trip, just two days before he is set to meet one of Mr. Trump’s biggest boosters, President Vladimir V. Putin of Russia.

In his remarks, Mr. Biden also took aim at Republican senators who he said “know better” about opposing an investigation into the Jan. 6 riot at the Capitol, saying many are doing so because they are worried about being opposed by a more conservative primary opponent.

But he said he did not feel the need to talk with foreign leaders about Mr. Trump’s grip on the Republican Party in the United States, because he does not believe the former president’s influence will hurt his ability to stand by his global commitments.

“The Republican Party is vastly diminished in numbers,” Mr. Biden said. “The leadership of the Republican Party is fractured and the Trump wing of the party is the bulk of the party, but it makes up a significant minority of the American people.”American officials in the past have typically made a point of leaving domestic politics behind when traveling abroad. But the outdated saying that politics stops at the water’s edge was blown to shred by Mr. Trump, who often used foreign trips to rail against his political enemies at home. Mr. Biden’s comments about the opposing party were tame, by comparison, but still marked an unusual breach of norms by a president intent on restoring them at home and abroad.

Mr. Biden’s optimism about the future of the Republican party is not shared by many in his party, who have publicly expressed fear that Republicans are increasingly in thrall to Mr. Trump, making them unwilling to take part in the give-and-take of governing in Washington.

And officials throughout Europe have said they are still braced for a return of Trumpism if Republicans take over the Congress in 2022 or if someone like Mr. Trump — or Mr. Trump himself — wins the White House again in 2024. That fear makes some political leaders around the world nervous about whether America’s long-term commitments can be sustained.

Mr. Biden said he did not worry about that.

“I’m not making any promises to anyone that I don’t believe are overwhelmingly likely to be kept,” he insisted.

He also said the Republican party seemed to be changing.

“I think you’re going to see that, God willing, we’re going to be making progress,” he said. “And there’s going to be a coalescing of a lot of Republicans, particularly younger Republicans, who are coming up in the party.”