As Russia prepared to strike Ukraine and the United States rushed to defend neighboring allies in Europe, former President Donald J. Trump had nothing but admiration for President Vladimir V. Putin of Russia.

He is “pretty smart,” Mr. Trump said on Wednesday at a Florida fund-raiser, assessing the impending invasion like a real estate deal. “He’s taken over a country for $2 worth of sanctions,” he said, “taking over a country — really a vast, vast location, a great piece of land with a lot of people — and just walking right in.”

Historians called the remarks unprecedented. “The idea that a former president would praise the man or leadership who American troops are even now traveling to confront and contain,” said Jeffrey Engel, a presidential historian at Southern Methodist University in Dallas, “is astounding.”

Yet Republican leaders, while condemning the invasion of Ukraine, stayed silent about the ex-president’s repeated praise this week for Mr. Putin, even as some Trump allies — from former administration figures to the Fox News host Tucker Carlson — amplified his Russian-friendly views to the party’s core.

Foreign policy experts and Russia scholars said the apparent sympathy or ambivalence toward Moscow from elements of the right raised questions about the influence Mr. Trump continues to exert over candidates seeking to tap into his base, the legacy of a decade-old effort by the Kremlin to court American conservatives and the future of the G.O.P. amid a backlash against the Republican-led entanglements in Afghanistan and Iraq.

Cold War-era Republicans, historians said, would have repudiated comments like Mr. Trump’s as un-American. Anders Stephanson, a historian of foreign policy at Columbia University, recalled an earlier Russian invasion. “Could one imagine Dwight Eisenhower praising Leonid Brezhnev for invading Czechoslovakia in 1968?” he asked in an email. “I think not.”

Republican congressional leaders on Thursday stayed far away from the Putin-friendly views that had been emanating from the former president. Senator Mitch McConnell of Kentucky, the Republican leader, denounced Mr. Putin at length and urged the Biden administration to provide military aid to help the Ukrainians fight back. Asked at a news conference in Louisville about Mr. Trump’s comments, the senator responded with silence.

As the threat of a Russian invasion rose, Mike Pompeo, a former secretary of state and C.I.A. chief under Mr. Trump, and now a potential presidential aspirant, appeared to take a similar line to his former boss. Mr. Putin was “an elegantly sophisticated counterpart” and “very shrewd,” Mr. Pompeo said.

But as the Russian onslaught neared, Mr. Pompeo on Wednesday qualified those views. Mr. Putin was also “evil” and “should be crushed,” he told The Des Moines Register on a visit to Iowa.

J.D. Vance, a venture capitalist and author who is courting Trump supporters as a candidate in the Ohio Republican primary for a Senate seat, recalibrated as well.

“I don’t really care what happens to Ukraine one way or the other,” Mr. Vance said a few days ago, aligning himself with the former president.

Yet on Thursday he issued a blistering statement both blaming “elites” for “the tragedy” of Russia’s invasion and opposing “further engulfing ourselves in Eastern Europe.”

“Putin is an evil man,” Mr. Vance said, “but the foreign policy establishment that led Ukraine to the slaughterhouse deserves nothing but scorn.”

Other Republicans across the country who are closest to the party’s base — House members and primary candidates — have often sought to deflect questions about their stance on Ukraine with answers that avoid breaking with Mr. Trump or agreeing with President Biden. A chorus of Republicans are arguing that the White House is worrying more about a distant conflict than about illegal immigration.

“Why does Joe Biden care more about Ukraine’s border than the U.S. southern border?” the official Twitter account of the Republican minority of the House Judiciary Committee declared on Wednesday.

Senator Josh Hawley, Republican of Missouri, last week took a step toward the Trump-supporting base and away from the Republican leadership, arguing that NATO should placate Russia by refraining from expanding membership to Ukraine.

“My view is that China is our number one security and economic threat,” Mr. Hawley said in an interview last week. “This is what leads me to being very skeptical of an expansion. I don’t think we can afford, in both the literal and colloquial uses of that term, to expand our security commitments in Europe.”

But on Thursday, Mr. Hawley joined the leadership in urging Mr. Biden to send Ukraine military equipment and “sanction Russian energy production to a halt.”

The former president’s allies in the conservative media, meanwhile, have carried his praise for Mr. Putin into a more fully formed argument against opposing the invasion of Ukraine. Tucker Carlson has echoed Kremlin talking points so closely that his sound bites have become a staple of Russian state television. 

Mr. Carlson, a onetime hawk who turned against American adventurism in the wake of the Iraq war, has openly questioned U.S. commitments to Ukraine since Mr. Trump’s first impeachment, when the former president was said to have withheld military aid to Ukraine to pressure officials there into investigating Mr. Biden and his family.

“What is this really about?” Mr. Carlson asked this week of the Biden administration’s condemnation of Russia. He answered himself with a series of questions inviting listeners to direct their anger at American liberals or at China instead. “Has Putin ever called me a racist?” Mr. Carlson asked, adding, “Did he manufacture a worldwide pandemic?” 

“We are always rooting for peace,” he insisted.

Andrew S. Weiss, a Russia expert at the Carnegie Endowment for International Peace, said the Kremlin had sought for a decade to win over allies on the American right, in part by denouncing gay rights, emphasizing Russian support for conservative social norms and inviting visits from prominent evangelical figures like Franklin Graham.

“It’s worked beautifully,” Mr. Weiss said.

Stephen K. Bannon, a former adviser to Mr. Trump who hosts a popular conservative podcast, hinted this week at the success of those efforts. “Putin ain’t woke — he is anti-woke,” Mr. Bannon said approvingly on Wednesday. He was interviewing Erik Prince, the private security contractor and a member of a prominent family of evangelical Christians and Republican donors, who joined Mr. Bannon in commending Russia for its opposition to transgender rights.

On Thursday, Mr. Bannon argued that Congress should impeach Mr. Biden for “instigating this war in Ukraine.”

“There is no appetite in Europe to defend themselves, OK?” Mr. Bannon said. “Now you’ve gone in and stirred up a hornet’s nest.”

Hal Brands, a historian at Johns Hopkins University’s School of Advanced International Studies and a resident scholar at the conservative American Enterprise Institute, compared the apparent sympathy for Russia among some on the right to earlier periods when groups on the political fringes embraced foreign rivals as foils for domestic opponents.

During the years before the United States entered World War II, for example, a handful of lawmakers lauded Adolf Hitler and Benito Mussolini for their strong leadership. During the early years of the Cold War, he noted, some on the far left spoke approvingly of the Soviet Union as an alternative to unfettered capitalism.

“Russia is a stand-in for anti-wokeness,” he said.

But the current “fascination with Putin” among some on the right, he added, “is also wrapped up in the post-Trump sweepstakes.”

“You see a lot of emulation going on among politicians who may or may not be authentically Trumpian but nonetheless want to claim that part of the party’s base for their own political ambitions.”

Nicholas Confessore contributed reporting.