WASHINGTON — The Senate Republican news conference on Wednesday was proceeding with the usual partisan criticism of President Biden and exhortations for him to do more — much more — to bolster Ukraine’s defense when the microphone went to Senator Ted Cruz.

The Texas Republican, in a made-for-television voice, made a stark assertion: “This war didn’t have to happen — the most significant war in Europe since 1945, since the end of World War II,” he said, before telling reporters that Mr. Biden’s White House “caused this.”

Lawmakers in both parties have described their shared determination to support Ukraine in its fight against Russia as the most remarkable consensus in Congress since the aftermath of the attacks of Sept. 11, 2001. “President Zelensky has managed not only to unite the West; to a large extent, he’s managed to unite the Congress,” Senator John Kennedy, Republican of Louisiana, said of Ukrainian President Volodymyr Zelensky.

But the sense of common purpose has not translated into bipartisan backing for the commander in chief; if anything, it has sharpened Republicans’ lines of attack against Mr. Biden.

Senator Jim Risch of Idaho, the lead Republican on the Foreign Relations Committee, emerged from Mr. Zelensky’s joint address to Congress on Wednesday to proclaim that the carnage depicted in a video that the Ukrainian president played for lawmakers was a direct result of a response by the Biden administration that had been “slow, too little, too late.”

Mr. Kennedy traced Russian President Vladimir V. Putin’s invasion back to the withdrawal of U.S. forces from Afghanistan, the failure to attack Syria after its leader used chemical weapons, and the Russian seizure of Crimea, all of which, he made sure to note, “happened when Joe Biden was either vice president or president.”

Absent from that analysis were four years under President Donald Trump during which he repeatedly undermined NATO, sided with Mr. Putin over his own intelligence community on Russia’s interference in the 2016 election and tried to bring Russia back into the community of developed economies. Also missing was Mr. Kennedy’s own trip, with seven other Senate Republicans, to the Kremlin on July 4, 2018, after a bipartisan report of the Senate Intelligence Committee determined that Moscow had interfered in the 2016 election on Mr. Trump’s behalf.

Senator Ron Johnson of Wisconsin was also on that trip to the Kremlin, then launched an investigation of Hunter Biden in Ukraine that sparked warnings by Democrats that he was serving as a conduit of Russian disinformation. Mr. Johnson told Fox News host Brian Kilmeade on Tuesday: “The problem we have dealing with these tyrants is the Democrats, the Biden administration, all their policies are weakening America.”

Democrats argue that such criticism shows how single-minded the Republican Party has become about tearing down its opponents.

“Republicans have defaulted to attacking Joe Biden in a moment of national crisis,” said Senator Christopher S. Murphy, Democrat of Connecticut. “There’s this infection in the Republican Party right now, in which power matters more than anything else, more than democracy, more than the peaceful transition of power, more than winning wars overseas.”

Some Republicans have taken a different line of attack. On the far-right fringe, Representative Marjorie Taylor Greene, Republican of Georgia, declared that an independent Ukraine only exists because the Obama administration “helped to overthrow the previous regime,” a reference to the popular uprising that took down a pro-Russian president of Ukraine — actually two Ukrainian governments ago.

She, too, blamed the Biden administration, but said she opposed any intervention. Another far-right Republican, Representative Madison Cawthorn of North Carolina, was videotaped calling Mr. Zelensky “a thug,” a comment that Russian propagandists continue to use.

On the other end of the spectrum, Senator Mitt Romney, Republican of Utah, offered a more comprehensive historical analysis.

“I wish we’d have armed Ukraine more than we did, but that’s true for not just Biden, but Trump and before him,” said Mr. Romney, who warned during the 2012 presidential debate of a looming threat from Russia. “But,” he added, “Vladimir Putin is responsible for what’s happened in Ukraine,” not Mr. Biden.

One Republican House member, speaking on condition of anonymity for fear of angering party leaders, said the war in Ukraine is likely to buoy the president’s standing with the public and could mitigate Democratic losses in the midterm elections.

Democrats have blamed inflation and rising gasoline prices — problems that predated the invasion of Ukraine — on Mr. Putin. The growing ferocity of Republican criticism could truncate any natural rallying around the flag.

But public opinion, three weeks into the war, is mixed. Nearly half of Americans, 47 percent, approve of the Biden administration’s handling of the crisis, while 39 percent disapprove, according to a new survey by the Pew Research Center. Opinion is even more divided on the U.S. role going forward: 42 percent say America should be providing more support to Ukraine, while 32 percent say the current level is about right. Just a sliver, 7 percent, take Ms. Greene’s position that the United States is already doing too much.

Richard H. Kohn, professor emeritus of peace, war, and defense at the University of North Carolina, Chapel Hill, noted that internal strife has been “vicious” in periods when war was raging but the United States was not engaged in combat, such as during the early years of the two world wars.

The political consensus at the start of the Cold War was shattered by Vietnam, when Senator Barry Goldwater articulated a view still dominant in the G.O.P., that the military should be all in or all out. The vaunted unity after 9/11 broke down 18 months later with the invasion of Iraq.

But, Prof. Kohn said, this moment has its particulars. Mr. Biden’s hard slide in public opinion polls began with the withdrawal from Afghanistan, and Republicans feel they need to keep up the pressure, especially on foreign policy.

And because public sympathy lies overwhelmingly with Ukraine, Republicans are searching for ways to distinguish their position from the president’s, turning minor issues like Poland’s offer to transfer its fleet of aging MIG fighter jets to Kyiv, which the Pentagon rejected, into matters of life or death.

Fully 83 percent of Americans now see Russia as an enemy, and the shift among Republicans is dramatic, from 50.6 percent in February 2017 who saw Russia as an enemy to 84.7 percent now, according to new research by YouGov.

“Republicans are grasping,” Prof. Kohn said.

Democrats have repeatedly countered Republican attacks by pointing to their silence when Mr. Trump bragged of his warm personal relations with Mr. Putin; when it was revealed that Mr. Trump’s company had been negotiating to build a skyscraper in Moscow well into the 2016 campaign, despite his outright denials; and when he stood by Mr. Putin’s side in Helsinki, Finland, and said he believed Mr. Putin’s denials over U.S. intelligence’s conclusion that the Kremlin had interfered in the 2016 election.

Every Senate Republican but Mr. Romney voted to acquit Mr. Trump of abusing his power when he withheld military aid to Ukraine, then demanded that Mr. Zelensky publicly announce an investigation of Mr. Biden and his son Hunter.

Only Representative Adam Kinzinger, Republican of Illinois, has publicly expressed remorse for failing to hold the former president accountable for a “shameful, illegal act” that weakened Mr. Zelensky as Russian-backed separatists were actively battling the Ukrainian military in eastern Ukraine.

Others have been less willing to reflect on their party’s role.

“We need to elevate this beyond that and start talking about what needs to be done from here, not what got us here,” Mr. Risch said. “Let the historians take it from here. They’ll clean this little mess up when it’s over.”

Mr. Cruz is adamant that Mr. Biden is to blame for Russia’s invasion. He argues that the president’s hasty, bloody withdrawal of forces from Afghanistan and his decision to waive sanctions against a natural gas pipeline from Russia to Western Europe caused the attack.

“Putin is a megalomaniacal dictator. Biden is the president who decided to lift sanctions on him and greenlight the project that Putin was building in order to enable him to invade Ukraine,” Mr. Cruz said.

Nicola De Blasio, a senior fellow at Harvard’s Belfer Center for Science and International Affairs and an expert on the Nord Stream 2 pipeline, said that argument was absurd. The pipeline was only expanding the capacity of an existing one that continues to operate.

“When American policymakers make a big fuss over Nord Stream 2,” he said, “I laugh.”

Mr. Cruz’s timeline also leaves out most of the previous administration, during which around 80 percent of the pipeline was built, Mr. De Blasio said. By the time the Senate failed to pass Mr. Cruz’s new round of sanctions on the Germans behind Nord Stream 2, Mr. De Blasio said, the pipeline was complete and simply awaiting administrative approval before being switched on.

What Mr. Cruz called “a generational geopolitical mistake” by Mr. Biden was, he said, irrelevant.

“You’re making a fuss about one pipeline when there’s another one already pumping gas,” Mr. De Blasio said.