When President Volodymyr Zelensky of Ukraine visited Capitol Hill less than two years ago, he was feted with standing-room-only crowds, rollicking ovations and an aid package worth nearly $50 billion to help his country fight off a Russian invasion.

His reception on Thursday was far more muted, as a few dozen lawmakers huddled with him behind closed doors while the speaker of the House snubbed him.

Mr. Zelensky is widely regarded as Ukraine’s most persuasive advocate, gifted in his ability to cut through partisan congressional gridlock with appeals to speed weapons and other supplies to Kyiv. But as the war against Russia drags on, his star power in Washington has noticeably faded, with potentially dire consequences for the future of U.S. military assistance to Ukraine.

Ukraine is working its way through a nearly $61 billion military aid package that Congress approved in the spring and that the Pentagon is trying to get out the door before President Biden leaves office. On Thursday, Mr. Biden announced that the United States would send Ukraine $8 billion worth of weapons, including glide bombs, air defense missiles and an additional Patriot battery.

But when that military assistance runs out, Ukraine’s fate will once again depend in large part on Congress’s willingness to keep replenishing Mr. Zelensky’s war chest.

Mr. Zelensky held two meetings at the Capitol on Thursday, first taking questions from a bipartisan group of about 20 senators, including Chuck Schumer, Democrat of New York and the majority leader, and Mitch McConnell, Republican of Kentucky and the minority leader. He then met with a bipartisan group of about a dozen House members, including Representative Hakeem Jeffries, the minority leader.