WASHINGTON — In the final years of Donald J. Trump’s presidency, Republicans portrayed Ukraine as an Eastern European Wild West run by nefarious oligarchs and unlawful politicians, a bad actor that sought to tamper in American elections and channel millions of dollars to Joseph R. Biden Jr.’s son.

“We’re talking Ukraine,” thundered Representative Jim Jordan, Republican of Ohio, in 2019, describing the country as “one of the three most corrupt countries on the planet.” The setting was a hearing for Mr. Trump’s first impeachment, over his efforts to pressure Ukraine’s president, Volodymyr Zelensky, into digging up political dirt on Mr. Biden.

Now such voices are fading, as the bulk of the Republican Party tries to get on the right side of history amid a brutal Russian invasion of Ukraine. Republicans are among the most vociferous champions for the United States to amp up its military response, and are competing to issue the strongest expressions of solidarity with Ukraine’s leaders.

Senator Roger Wicker of Mississippi has taken up Mr. Zelensky’s call for a western-enforced no-fly zone. Senator Rick Scott of Florida said deploying U.S. ground troops to Ukraine should not be “off the table.” Senator Lindsey Graham of South Carolina encouraged the assassination of President Vladimir V. Putin of Russia to save a nation that many in his party had previously portrayed as hardly worth saving.

“What was sort of a problematic, corrupt place is now the defender of freedom,” Mr. Graham quipped about his colleagues’ changing tunes.

The Republican center of gravity has undergone what Senator Richard Blumenthal, Democrat of Connecticut and a longtime advocate for the Ukrainian community in his state, called a “sea change,” a swing of the pendulum so sharp that some fear it could lead Congress to unwittingly widen the war.

“I am extremely concerned that some of these policies being pushed from policymakers on both sides of aisle will put us on a path toward a conflict with a nuclear-armed Russia,” said Dan Caldwell, the vice president of foreign policy for Stand Together, a group funded by the conservative billionaire Charles Koch that advocates military restraint.

In January, as Russia continued to amass more troops near Ukraine’s borders, Republicans including Representatives Matt Rosendale of Montana, Lauren Boebert of Colorado, Paul Gosar of Arizona, Thomas Massie of Kentucky, and Marjorie Taylor Greene of Georgia weighed in to oppose the United States confronting Russia or to suggest that President Biden had malevolent intentions in his handling of the matter. So did the Ohio Senate candidate J.D. Vance and Donald Trump Jr.

Now, even the far-right flank seems confused. On Monday, Ms. Greene used her Twitter account to both call one of the whistle-blowers in former President Trump’s first impeachment, retired Lt. Col. Alexander S. Vindman, a “clown” who was “clueless about Americans being fed up with sending our sons and daughters to die in foreign lands,” and advise, “While innocent people are being murdered in Putin’s war on Ukraine, the U.S. response is critical.”

Senator Tom Cotton, Republican of Arkansas, gave a lengthy speech on Monday at the Ronald Reagan President Library in California, trying to explain how the party of Reagan, with its aggressive stand against the Soviet Union, could be the same as the party of Trump, which stood by as a president sided with Russia’s autocratic president against the judgment of America’s intelligence community.

“Vladimir Putin must pay for this unprovoked, naked war of aggression,” he concluded. “If Joe Biden won’t make him pay, the Republican Party must.”

The Fox News personality Tucker Carlson, one of the last loud voices for isolationism from the conflict, expressed frustration on Monday night with the Republican pivot, saying the party is now more pro-war than Mr. Biden.

“The ground has definitely shifted,” Mr. Blumenthal said. “Remember, they not only thought of Ukraine as at the bottom of the list of friendly nations for the U.S., but they also put Putin at the top of supposedly friendly leaders.”

For a distant country on the edge of what was once the Soviet empire, Ukraine has played an outsized role in American politics for a half dozen years. One of Mr. Trump’s 2016 campaign managers, Paul Manafort, lobbied on behalf of the country’s ousted pro-Russia leader, pressing ahead of the Republican National Convention in 2016 to shift the party’s platform away from its support for the country.

Mr. Trump and his supporters tried to deflect attention from Russia’s interference on his behalf in the 2016 election by accusing Ukraine of interfering on Hillary Clinton’s behalf.

Mr. Trump’s first impeachment grew out of his effort to withhold military support from Ukraine until Mr. Zelensky agreed to investigate Mr. Biden and his son, Hunter, and their involvement in a Ukrainian energy company, Burisma.

After every Senate Republican except Mitt Romney of Utah voted to acquit Mr. Trump, Senators Ron Johnson, Republican of Wisconsin, and Charles E. Grassley, Republican of Iowa, turned the tables and began a formal investigation into whether Mr. Biden, as vice president, had tried to pressure Ukrainian authorities to protect his son, who was on the Burisma board.

Even some Republicans at the time, like Mr. Graham and Senator Richard M. Burr of North Carolina, worried that the Johnson-Grassley investigation was being used by Russia to spread disinformation ahead of the 2020 election. The inquiry ultimately found no evidence of wrongdoing.

Now, that investigation is a distant memory, and Republicans are working to explain their stark change in perspective.

“Corruption’s with politicians most of the time, or with big business people,” Mr. Grassley said on Tuesday. “In this case, you’ve got children being killed by bombs. There’s no connection.”

Senator Christopher S. Murphy, Democrat of Connecticut and another longtime defender of Ukraine, noted that for much of the Trump administration, Senate Republicans quietly bolstered Ukraine’s defenses and backed Mr. Zelensky, even as they remained silent on Mr. Trump’s undermining of the Zelensky government. Mr. Trump was withholding lethal military aid that Congress had provided to Ukraine with broad bipartisan support.

“I wish they would have made clear that there should be consequences to a president who uses Ukraine as a political chess piece,” he said. “There’s been a subset of Republicans that are willing to use Ukraine as a political piece in whatever narrative they’re trying to spin to win elections.”

“But,” he added, “that’s a subset of Republicans, not necessarily representative of the broader caucus.”

Representative Steve Scalise of Louisiana, the No. 2 House Republican, resurrected talking points from the first Trump impeachment on Tuesday, insisting misleadingly that it was Mr. Biden who had withheld aid, while Mr. Trump had given anti-tank weapons to Ukraine after the Obama administration had refused. (In 2016, Mr. Biden, then the vice president, threatened to withhold loan guarantees from Ukraine unless a corrupt prosecutor was fired, in line with a broader international effort to oust that official, but did not ultimately hold back any resources.)

During a now-infamous 2019 phone call that Mr. Trump had with his Ukrainian counterpart, Mr. Zelensky had raised purchasing those anti-tank missiles when Mr. Trump interjected, “I would like you to do us a favor though,” before launching into a series of politically charged requests.

“President Trump stood with President Zelensky and in fact, the two of them had a really good relationship,” Mr. Scalise said.

Senator John Thune of South Dakota, the No. 2 Republican in the Senate, did not try to explain away the inconsistencies.

“There’s a lot of things that happened in the past, none of which I think it makes a lot of sense to relitigate,” he said. “I just think we have to play the hand we’re dealt and do everything we can right now to try and stop this thing, and that means getting all the aid we can to the Ukrainian people to fight the good fight.”

Annie Karni contributed reporting.