Recent fighting in the House has highlighted a drop in Republican support for aid to Kyiv.

The Republican meltdown on Capitol Hill that toppled the speaker this week and left the House in chaos has also highlighted a sharp decline in G.O.P. support for continuing to send aid to Ukraine, and how opposition to helping Kyiv has become a litmus test for the right.

The intensifying shift is striking for a party that has long defined itself by its belief in a muscular American military defending democracy around the world. And it could make it far more difficult for the Biden administration to fulfill its promise to support Ukrainian fighters for the long haul.

Hard-line Republican critics have long espoused isolationist views about Ukraine’s war effort, arguing that sending tens of billions of dollars to Kyiv risks dragging the United States into a head-on conflict with Russia and siphons money away from domestic challenges. Former President Donald J. Trump popularized the argument with his “America First” approach to foreign policy, but until recently, most lawmakers refrained from embracing it.

But the drama that has played out in the House over the last week, as Republicans pushed the government to the brink of a shutdown and then deposed their own speaker, has made clear that the right-wing message is gaining momentum among Republicans.

In the past few days, Republicans managed to strip billions in military and humanitarian assistance requested by Mr. Biden out of a stopgap spending bill to keep the government from shutting down. They rallied a majority of their colleagues in the House to vote against funding a program to train and equip Ukrainian troops. And a small faction of hard-liners joined with Democrats to boot Representative Kevin McCarthy, Republican of California, as speaker after accusing him of making a “secret side deal” with Mr. Biden to fund Kyiv’s war against Russia.

The division among Republicans on the issue is now on vivid display in the fight to replace Mr. McCarthy, which pits Representative Steve Scalise of Louisiana, the No. 2 Republican who has backed aid to Ukraine, against Representative Jim Jordan of Ohio, the Judiciary Committee chairman who is vocally opposed.

While the naysayers still represent a minority overall in Congress, the dramatic shift in Republican sentiment has left Ukraine’s boosters in the party angry, alarmed and working to figure out how to reverse the trend before a lapse in funding hampers Ukraine on the battlefield.