WASHINGTON — The first time that Senator John Kennedy, Republican of Louisiana, went on a Sunday television talk show and suggested that it might have been Ukraine, rather than Russia, that interfered in the 2016 election, he drew so much criticism that he quickly apologized.
But the following week, Mr. Kennedy was back on the Sunday circuit, returning to a claim that contradicted the findings of American intelligence officials, who long ago concluded that Moscow attacked the election, and who recently told congressional officials that Russia had been engaged in a disinformation campaign to frame Ukraine for it.
And this week, as the Senate returned from its Thanksgiving recess facing the prospect of an impeachment trial in the new year, some other Republican senators have joined Mr. Kennedy in latching on to the theory, testing out a pivot as they attempt to mount a more vigorous defense of the president.
The issue is crucial to the impeachment story, because it mirrors a claim that Mr. Trump made — and asked President Volodymyr Zelensky of Ukraine to investigate — in a July phone call that sparked the inquiry. Democrats argue that the president’s bid to pressure Ukraine to announce that investigation, and another into former Vice President Joseph R. Biden Jr., was a dangerous abuse of power for his own political gain, and grounds for impeachment. Republicans are toiling to put the request in a more positive light.
Holding court with reporters in the Senate basement on Monday armed with a stack of articles he said vindicated his assertion, Mr. Kennedy had an unequivocal answer to the question of whether he believed Ukraine interfered in the election.
“I do,” Mr. Kennedy said. “And I’m not the only one.”
Most Republican lawmakers have backed away from the theory. But Senator Richard M. Burr of North Carolina, the chairman of the Intelligence Committee, told reporters on Tuesday that there was “no difference” between the far-reaching Russian interference his panel documented over the course of a multiyear investigation and Ukrainian officials who openly preferred Hillary Clinton’s candidacy over Mr. Trump’s.
The statement was all the more remarkable because his panel investigated the claim, according to a congressional official with knowledge of the inquiry, and could find no evidence that Ukraine engaged in an interference campaign similar to Russia’s.
Mr. Burr had alluded to that finding in a statement to CNN on Monday night: “I don’t think anybody interfered in the same way Russia did.”
Senator Mitch McConnell of Kentucky, the majority leader, declined to answer whether Ukraine interfered in the election, arguing first that it was a matter for the House to address and then that it was up to the House or Senate Intelligence Committee to investigate.
Their equivocations were the latest example of how Republicans have contorted themselves to embrace Mr. Trump’s narrative on the Ukraine matter. And the president has clearly appreciated the efforts.
“Thank you to Great Republican @SenJohnKennedy for the job he did in representing both the Republican Party and myself against Sleepy Eyes Chuck Todd on Meet the Depressed!” Mr. Trump wrote on Twitter after Mr. Kennedy, referring to the senator’s airing of the Ukraine theory on NBC’s “Meet the Press.”
Senior administration officials were less appreciative.
David Hale, the No. 3 State Department official, testified on Tuesday before the Senate Foreign Relations Committee that he had not seen any evidence to suggest Ukraine interfered in the election.
Asked by Senator Robert Menendez of New Jersey, the top Democrat on the panel, whether lawmakers repeating the claim were making the nation’s security “stronger or weaker,” he offered a terse answer: “It does not serve our interests.”
Senator Lindsey Graham, Republican of South Carolina and the chairman of the Judiciary Committee, appeared to agree. Mr. Graham said on Tuesday that he was “1,000 percent confident” that the hack of the Democratic National Committee “was by Russian operatives, no one else.”
“I think it’s always wrong to say things that can’t be proven,” Mr. Graham added. “It was the Russians.”
Senator John Thune of South Dakota, the No. 2 Republican, also pushed back. “Everything I’ve seen in the intelligence community and in our Intelligence Committee puts it squarely on Russia,” he told reporters.
And Senator Mitt Romney of Utah, a member of the Foreign Relations Committee who is among the president’s most vocal Republican critics, said there was no comparison between support for Mr. Trump’s opponent by some Ukrainian officials and the large-scale effort carried out by Russia’s government to subvert the election.
“There’s a big difference between pulling for someone and hoping someone wins in the American election and interfering the way that Russia did,” Mr. Romney said.
Democrats on Tuesday were scathing in their rebuke of the Republican embrace of the Ukraine claim. Senator Chuck Schumer of New York, the minority leader, said it was “appalling” that Republican lawmakers were “repeating the lie invented by” Russian intelligence services.
“I have a simple message for my Republican colleagues,” Mr. Schumer said. “Stop spreading Putin’s propaganda.”
Senator Mark Warner of Virginia, the top Democrat on the Intelligence Committee, also weighed in.
“There is absolutely no factual basis for this Ukrainian election interference/CrowdStrike nonsense,” Mr. Warner said on Twitter, referring to a cybersecurity firm that examined the Democratic National Committee’s server and has been pulled into the conspiracy theory. “None.”
Mr. Kennedy, an Oxford-educated lawyer who is perhaps best known in the Senate for his folksy aphorisms and pointed grilling of judicial nominees, has emerged as one of Mr. Trump’s most vigorous impeachment defenders in the Senate. He first floated the theory about Ukraine’s culpability more than a week ago on Fox News, saying that the perpetrator of the hack on a server at the Democratic National Committee “could be Ukraine.”
The next day, he told CNN that he “was wrong” and that the “only evidence” he had “is that it was Russia who tried to hack the D.N.C. computer.”
But on Sunday, he reverted to his initial claim on “Meet the Press.”
Sheryl Gay Stolberg contributed reporting.