At stake is $24 billion requested by Biden to assist the war-torn nation.
Ukrainian President Volodymyr Zelenskyy came Washington on Thursday to make the case for $24 billion more in U.S. tied up in a showdown between House Republicans over spending.
Zelenskyy's first stop was Capitol Hill, where he was lobbying lawmakers behind closed doors. The Ukrainian leader was escorted by House Democratic leader Hakeem Jeffries and later flanked by Senate Majority Leader Chuck Schumer and Senate Minority Leader Mitch McConnell on their way to a meeting in the Old Senate chamber.
The aid he's pushing for was requested by President Joe Biden, and he, many congressional Republicans and most Democrats call it vital to Ukraine's efforts to repel Russia's invasion.
Zelenskyy's first visit to Washington last year helped him secure a $50 billion package, but since then a growing number of House Republicans have mounted resistance to Biden's request for more. Polls, too, have shown American support for Ukraine waning as the 19-month war continues.
Exiting his meeting with senators, Zelenskyy told reporters they had a "great dialogue" and was "very thankful" to the U.S. for its support.
"So many details but it will be between us," Zelenskyy said.
House Speaker Kevin McCarthy, who is trying to corral his caucus to consensus on spending before a Sept. 30 government shutdown deadline, declined to commit to providing such aid ahead of Zelenskyy's visit. Instead, he told reporters he had questions for the foreign leader.
"What is the plan for victory? Where are we currently on the field? The accountability issues that a lot of members have," McCarthy said as he arrived at the Capitol on Thursday.
Republican Rep. Mike McCaul, the chairman of the House Foreign Affairs Committee, said McCarthy raised those points during Zelenskyy's meeting with House members.
McCaul also said Zelenskyy discussed the $24 billion aid request and expressed confidence Ukraine would receive it despite Republican hard-liners being adamantly opposed.
"They did and they're gonna get it," McCaul said. "I said, you know, the majority of the majority support this. I know, there's some dissension on both sides, but I said war of attrition is not going to win this. That's what Putin wants, because he wants to break the will of the American people and the Europeans. We can't afford a war of attrition."
President Biden, in a speech at the United Nations General Assembly earlier this week, urged world leaders to stand by Ukraine amid Russia's aggression.
"We have to stand up to this naked aggression today and deter other would-be aggressors tomorrow," Biden said. "That's why the United States, together with our allies and partners around the world, will continue to stand with the brave people of Ukraine as they defend their sovereignty and territorial integrity and their freedom."
White House spokesman John Kirby said he believed it was important for members of Congress to hear directly from Zelenskyy about what his country is facing and what they need to combat Russian invaders.
"He is the best messenger for the people of Ukraine and his armed forces, clearly," Kirby said.
Zelenskyy's previous visit to Washington included a joint meeting of Congress, where he received standing ovations as he made a passionate plea for more support.
McCarthy said Zelenskyy asked for a joint session of Congress for this visit as well, but they didn't have "time" to accommodate him.
"What we're doing for for Zelenskyy is exactly the same thing we did for the prime minister of the UK, the prime minister of Italy," he said. "We'll put in the bipartisan group of members together to be there, no different than we do with anybody else. And this is a little busy week. We're dealing with funding issue. I don't know how we can slip that in in such a short time."
Biden and Zelenskyy will meet in the Oval Office Thursday afternoon. There, Biden is expected to announce a new military aid package for Ukraine worth hundreds of millions of dollars.
According to a U.S. official, the package will include capabilities to bolster Ukraine's air defenses as well as artillery ammunition and anti-armor capabilities. However, it's unclear if the package will include ATACMS long-range missiles -- a weapon that would allow Ukraine to reach targets up to 190 miles away.
Zelenskyy will also make an appearance at the Pentagon, where he will meet with Defense Secretary Lloyd Austin, chairman of the Joint Chiefs of Staff Gen. Mark Milley and other leaders.
ABC's Ben Gittleson contributed to this report.