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.

But as President Trump makes a point of pursuing friendlier relations with Moscow — a pivot that recently included accusing Ukraine’s president, Volodymyr Zelensky, of being a “dictator” and even suggesting that Ukraine started the war — few Republicans in Congress have been willing to denounce the shift.

Even some of those who did criticize Mr. Trump for his administration’s alignment with Russia at the United Nations this week did so without calling out the president or his advisers by name.

“Refusing to acknowledge Russia as the undeniable and unprovoked aggressor is more than an unseemly moral equivalency — it reflects a gross misunderstanding of the nature of negotiations and leverage,” Senator Mitch McConnell, Republican of Kentucky, wrote in a statement Monday. He blamed the Biden administration in part for the present state of affairs with Mr. Trump, arguing that President Joseph R. Biden Jr..’s “shameful hesitation and half-measures threaten to give way to something even more disgraceful: the obstinate denial of America’s security interest in Ukraine’s success.”

Senator Bill Cassidy, Republican of Louisiana, used the words of a judge at the Nuremberg Trials in 1946, in which an international military tribunal prosecuted Nazi officials after World War II, to express his disapproval of the Trump administration’s U.N. votes.

“To initiate a war of aggression, therefore, is not only an international crime; it is the supreme international crime differing only from other war crimes in that it contains within itself the accumulated evil of the whole,” he wrote on social media.