Ukrainian President Volodymyr Zelenskyy came to Washington on Thursday to make the case for $24 billion more in U.S. aid 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, wearing military green fatigues, 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 thanked the U.S. for its support.

"We talked about a lot of different missions," Zelenskyy said. "We were happy that senators and Congress, the White House, really the United States, of course the people of America, support us, supported from the first day of full-scale war and now together with us and we spoke about everything about support about the situation on the battlefield about our plans."

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 expressed confidence Ukraine would receive the funding despite Republican hard-liners being adamantly opposed. So far, McCarthy hasn't been wiling to bring up bipartisan funding legislation that would include Ukraine aid for a vote.

"We will get it done," McCaul vowed on the aid. "There are a lot of political machinations right now, but we are going to get it passed."

Zelenskyy next visited the Pentagon, where he met with Defense Secretary Lloyd Austin, chairman of the Joint Chiefs of Staff Gen. Mark Milley and other leaders.

Zelenskyy was greeted with an honor cordon on the Pentagon steps. He also took part in wreath-laying ceremony at the Pentagon's 9/11 memorial.

To cap off the visit, Zelenskyy will meet with Biden in the Oval Office on Thursday afternoon. 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."

In the White House meeting, 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.

McCaul said Zelenskyy repeated his request for ATACMS and F-16 fighter jets during his meeting with House members, and criticized the administration for withholding key systems. He called on Congress to include language in government funding bills to direct the administration to heed those requests.

"The weapons that he asked for, that this administration won't give, we write that into our appropriations bill, and I think the Democrats at the table ... we all agree," McCaul said.

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."

ABC's Ben Gittleson contributed to this report.