A band of moderate Republicans in Congress sharply criticized the Trump administration this week for siding with Russia at the United Nations on resolutions regarding the war in Ukraine, even as the majority of the G.O.P. turned a blind eye to the United States’ sudden embrace of a longtime adversary.

“The Trump Administration royally screwed up today on Ukraine,” Representative Don Bacon, Republican of Nebraska, wrote on social media on Monday night. “The vast majority of Americans stand up for independence, freedom and free markets, and against the bully and invader.”

The reproach from Mr. Bacon and others came after the United States on Monday voted against a U.N. General Assembly resolution, introduced by Ukraine, that condemned Russia for invading Ukrainian territory and demanded that it withdraw and face repercussions for war crimes. Shortly afterward, the United States succeeded in passing a U.N. Security Council resolution calling for peace in Ukraine that did not chastise Russia. Key U.S. allies, including Britain and France, abstained.

“This posture is a dramatic shift from American ideals of freedom and democracy,” Senator John Curtis, Republican of Utah, wrote on social media on Monday night, saying he was “deeply troubled by the vote,” which had “put us on the same side as Russia and North Korea.”

The Russian representative to the United Nations welcomed the move by the United States and said the new stance gave Moscow “a certain optimism” about the future of European security.

The position adopted by the Trump administration would have been anathema in recent years, during which the United States shipped arms to help Ukraine’s fighters beat back Russia’s invasion and sent funds to prop up the war-torn country’s civilian infrastructure. Even many Republicans who bristled at the tens of billions of dollars in aid packages that Congress approved for Ukraine were resolute that Russia and Vladimir V. Putin, its president, were the aggressors in the conflict.