WASHINGTON — As President Biden tries to forge a united allied response to Russian aggression in Ukraine, unity on the home front is strained by a Republican Party torn between traditional hawks in the leadership and a wing still loyal to Donald J. Trump’s isolationist instincts and pro-Russian sentiment.

Republican leaders, by and large, have struck an aggressive posture, encouraging Mr. Biden to get tougher on Russia, through immediate sanctions on Russian energy exports and more lethal aid to Ukraine’s military. But that message has been undermined by the party’s far right, which has questioned why the United States would side with Ukraine at all, and obliquely suggested with no evidence that the president is bolstering his son Hunter Biden’s business interests.

Driven by a steady diet of pro-Russian or anti-interventionist rhetoric from the Fox News host Tucker Carlson, the Republican right has become increasingly vocal in undercutting not only U.S. foreign policy but also the positions of Republican leaders.

Representatives Matt Rosendale, Republican of Montana, Lauren Boebert, Republican of Colorado, Paul Gosar, Republican of Arizona, Thomas Massie, Republican of Kentucky, Marjorie Taylor Greene, Republican of Georgia, Donald Trump Jr. and the Ohio Senate candidate J.D. Vance have weighed in to oppose confronting Russia or to suggest nefarious intentions on Mr. Biden’s part. Mr. Trump told the conservative podcast host Lou Dobbs that Mr. Biden’s reported plan to send as many as 50,000 troops to bolster Europe’s defenses was “crazy.”

Representative Mike Turner of Ohio, the top Republican on the House Intelligence Committee, went on Fox News to confront Mr. Carlson.

“Why would we take Ukraine’s side and not Russia’s side?” Mr. Carlson pushed. “It’s a sincere question.”

Mr. Turner responded: “Ukraine is a democracy. Russia is an authoritarian regime that is seeking to impose its will upon a validly elected democracy in Ukraine, and we’re on the side of democracy.”

Such confrontations have muddied the Republican response, but more concerning are worries that the right could prompt U.S. allies to question Washington’s resolve. On a conference call on Wednesday with Ukrainian Americans, Senator Richard Blumenthal, Democrat of Connecticut, expressed alarm about “the rhetoric of the far right.”

“Our allies are watching closely for signs of division here,” he said in an interview after the call. “The good news is, I detect no real traction of those messages from my Republican colleagues.”

The bad news, he allowed, is that most Republicans are willing to disown it only in private.

Senator Chris Coons, Democrat of Delaware, said legislation to impose sanctions on Russia and bolster Ukraine’s military and economic position could be on the Senate floor as soon as next week, and he hopes it will counter any message of division.

“This is a moment for us to come together and pass a strong, bipartisan sanctions package to send an unmistakable signal of support for Ukraine, Ukraine’s independence and for President Biden’s leadership,” he said on Wednesday.

Publicly, Republican leaders have been talking tough. After Mr. Biden’s gaffe last week, when he seemed to suggest that a “minor incursion” into Ukraine would not merit a forceful allied response, Representative Steve Scalise of Louisiana, the No. 2 Republican, asked: “Do you think the strong, wonderful people of the Ukraine think it would be a minor incursion if Putin moved tanks into Ukraine, even a piece of the Ukraine? Of course they don’t.”

This week, speaking to reporters in Kentucky, the Senate Republican leader, Mitch McConnell, indicated such criticism had pushed the administration to toughen its stance.

“What I’ve been hearing since then is encouraging, that they’re prepared to take steps before an incursion, not afterwards,” he said, adding, “It appears to me the administration is moving in the right direction.”

But that direction — and that message — may not be what the most partisan Republican voters want. A Yahoo News/YouGov poll released this week found that 62 percent of Republicans and Republican-leaning independents view President Vladimir V. Putin of Russia as a stronger leader than Mr. Biden. But a survey released on Wednesday by the Pew Research Center found views of Russia’s military buildup near Ukraine did not differ much by partisan affiliation at all.

Representative Tom Malinowski, Democrat of New Jersey, said on Wednesday that some callers into his district office had begun parroting Mr. Carlson’s assertions that the United States should be allied with Russia, not Ukraine, or that he should be supporting Russia’s “reasonable” demands for NATO withdrawal from Eastern Europe.

“Most Republicans in the House are totally solid; to the extent they want Biden to do more, that’s at least a healthy instinct,” Mr. Malinowski said on Wednesday in a phone interview from Brussels, where a bipartisan House delegation is visiting NATO after meetings in Ukraine. “I do think some of them are slightly in denial about this other force in their party.”

For now, Mr. Malinowski said, the United States’ allies in Europe are united in their response to Russian aggression. Though they remain wary of American constancy after four years of Mr. Trump’s anti-NATO rhetoric, so far they trust that the Biden administration and Republican leadership will stand by the alliance if Russian forces invade Ukraine.

But, he warned, Republican leaders should not be fooled into thinking their foreign policy views could survive a resurgence of Trumpism in the White House, either with a return of the former president or the election of one of his acolytes. He pointed to the memo drafted in the closing days of the Trump White House to justify the firing of Defense Secretary Mark T. Esper. Those reasons included his focus on Russia and “every facet of competition” with Moscow, and his opposition to withdrawing U.S. troops from Germany.

In some sense, the taunts, insults and isolationist sentiments coming from the Republicans’ far-right flank are consistent with the Trump era, when Mr. Trump stood beside Mr. Putin in Helsinki, Finland, and said he trusted the Russian strongman over his own intelligence agencies.

“Despite claims by war hawks on both sides of the aisle, it is not in our national interest to spill American blood and treasure in Ukraine,” Mr. Rosendale wrote in a statement.

Mr. Gosar wrote: “We have no dog in the Ukraine fight. Not one American soldier should die there. Not one American bullet should be fired there. We just lost Afghanistan to sandal wearing goat herders. I assure you Russian military is no joke either.”

Others were simply insulting. Ms. Greene again called for Mr. Biden’s impeachment. Ms. Boebert brought up Burisma, the Ukrainian company that Hunter Biden did work for, leading to outlandish accusations of corruption from Republicans, which were later dispelled by a Senate Republican inquiry that found no evidence of improper influence or wrongdoing by the elder Mr. Biden.

Just as significant, some said on Wednesday, is the silence from Republicans, who have refused to speak out against their fringe.

Senator Christopher S. Murphy, Democrat of Connecticut, told Ukrainian Americans on Wednesday that a group of senators who just returned from Ukraine had been working for three days on a tough new bill that would impose broad sanctions on Russia.

That legislation would be a vehicle to show “there’s no daylight between Republicans and Democrats when it comes to the fundamental questions of whether we should provide significant economic and security support to Ukraine right now at unprecedented levels in order to make sure that Ukraine can defend itself,” he said. “And there is no daylight between Republicans and Democrats as to whether Putin should and will pay a devastating price if he were to launch an invasion.”