WASHINGTON — Hours after meeting in Kyiv with President Volodymyr Zelensky, Senator Mitch McConnell issued a forceful counterargument to the isolationists in his party who have questioned whether the United States should be sending another $40 billion in aid to Ukraine.
“This is not some handout,” Mr. McConnell, Republican of Kentucky, declared, in comments that could just as easily have been directed at the lawmakers he leads.
“It’s important for the United States to help; it’s important for the free world to help,” Mr. McConnell, the minority leader, told reporters. “It’s important for the Ukrainians to win, and hopefully not many members of my party will choose to politicize this issue.”
For years, Mr. McConnell has served as a bulwark against the anti-interventionist voices in the Republican Party that have grown ever louder, anchoring it to its hawkish roots even as President Donald J. Trump rejected them. Russia’s assault on Ukraine has presented perhaps his most difficult task yet, as he fights to stem the rising tide of isolationism in the G.O.P. and marshal support in Congress for sending billions of dollars in urgently needed military and humanitarian aid to the war-torn country.
Mr. McConnell’s efforts, for now, appear to have been successful. On Monday, only 11 Republicans voted against considering the aid package, in a lopsided test vote that demonstrated the overwhelming support in both parties for the measure.
But the mounting unease in Republican ranks with the enormous bill, once unthinkable, reflects how a significant segment of the party’s foreign policy shifted in lock step with Mr. Trump’s “America First” credo — and how it has retained a foothold even in the face of a brutal military campaign defined by programmatic violence against civilians.
Last week, 57 House Republicans voted against the aid package after conservatives including Donald Trump Jr., the former president’s eldest son, and the president of the Heritage Foundation lobbied hard against it.
“A lot of the voices in the media are going after this, and going after people, and it’s an election season,” said Senator John Thune of South Dakota, the Republican whip. “So people are obviously paying a lot of attention to what their constituents think. But I think you can defend this vote as a vote that is very vital to America’s national security interest.”
Cracks in the Senate Republican conference began to show days after their colleagues in the House revolted. Senator Bill Hagerty, Republican of Tennessee and Mr. Trump’s former ambassador to Japan, announced that he would oppose the bill, arguing that lawmakers were “rushing to take care of problems overseas” before resolving those at home.
“I don’t think America has anything against Ukraine,” Mr. Hagerty said on Fox News. “We don’t want to see them fail, but we have problems right here at home that we need to be paying attention to.”
Senator Roger Marshall, Republican of Kansas, said in a brief interview that he had voted against the aid package because $40 billion on top of the $13 billion “we already spent is just too much right now, too much all at one time.”
What people in his state are really concerned about, he said, is the southern border.
Both privately and publicly, Mr. McConnell has made the case that failing to stop President Vladimir V. Putin’s campaign in Ukraine would upend the international security order and pose a grave threat to the United States’ security. He made a similar argument in 2014, when he pressed for the United States to send aid to Kyiv as Mr. Putin invaded Crimea.
“This is not charity we’re involved in here,” he said on Sunday. “This is our interest — to help Ukrainians. Just like it is in the interest of NATO countries. This is not some handout. This is to prevent this ruthless thug from beginning a march through Europe.”
Behind closed doors, Mr. McConnell sought to bolster the Ukrainian government early in Russia’s invasion, his allies said, making the case himself and inviting top Ukrainian officials, including the ambassador to the United States, Oksana Markarova, to speak to his conference.
“His message very early was, ‘We need to get the Ukrainians anything they need, as quick as we can get it to them,’ ” said Senator Roy Blunt, Republican of Missouri. “I think generally, the Congress is very receptive to helping people fight for freedom, and I think Senator McConnell got there very early.”
But whether Mr. McConnell will be able to maintain the support among Republicans remains to be seen.
Kevin Roberts, the president of the Heritage Foundation, was strident on Monday before the test vote, releasing a statement titled “‘Manipulative Rhetoric’ Doesn’t Change Faults in Spending Proposal.”
“We do not need to choose between governing responsibly and advancing American interests on the world stage, no matter how much manipulative rhetoric is coming from Capitol Hill trying to convince us otherwise,” Mr. Roberts said.
Republicans running for Senate seats in several states have also expressed their opposition to the aid package. J.D. Vance, the Republican nominee for the Senate in Ohio, accused his Democratic opponent, Representative Tim Ryan, of “pushing billions in foreign aid while the communities he serves in Congress have been decimated,” attaching a Tucker Carlson clip for emphasis.
Adam Laxalt, a Republican challenging Senator Catherine Cortez Masto, Democrat of Nevada, called on her to “reject this shockingly abhorrent proposal.”
And Kathy Barnette, who is running for the Republican nomination for the Senate in Pennsylvania, dinged Mr. McConnell personally for visiting Kyiv.
“Why is Leader McConnell visiting Ukraine in the midst of the various crises right here in America?” she wrote on Twitter.
“I think it is almost guaranteed that this aid bill will pass,” said Dan Caldwell, the vice president for foreign policy at Stand Together, a group funded by the conservative billionaire Charles Koch that advocates military restraint. “But the fact that you have had a significantly larger group of congressional Republicans questioning the Biden administration’s strategy in Ukraine over the last week shows that there is an increasing wariness of open-ended support.”
Emily Cochrane contributed reporting.