Speaker Paul D. Ryan echoed the sentiments in a news conference. “Prime Minister Turnbull was in my office a couple months ago,” he said. “He’s a very important ally. Australia is a very central ally, they are and they will continue to be.”

Mr. Trump has broken with the policy positions of congressional Republicans on myriad issues, from trade to immigration to tax reform, and they have largely, so far, bent in his direction.

But in the area of foreign relations, particularly when it comes to longstanding American allies like Australia, Democrats and Republicans have been unified in discontent, and sometimes horror. They were displeased with Mr. Trump’s statements endorsing torture. They were made anxious by President Trump insulting the Iranian government on Twitter, seemingly his preferred medium for foreign confrontations.

They have routinely pushed back on his suggestions that the North Atlantic Treaty Organization and the European Union are outdated. The phone fracas with the leader of Australia was merely the latest. “The president says, ‘America First,’ and I agree,” said Senator John Cornyn of Texas, the No. 2 Republican in his chamber. “But obviously we have important allies around the world.”

House Republicans met last week at their policy retreat in Philadelphia with Theresa May, the British prime minister, before she headed to the White House, in part to convey an important message about America’s commitment to NATO.

While such conversations were once box-checking affairs, they have now become a matter of mild emergency, said several House and Senate aides. The Senate majority leader, Mitch McConnell, Republican of Kentucky, is particularly invested in international relations and is all but certain to provide quiet reassurance to many diplomats and world leaders.

Mr. Trump’s positions on Russia, in particular, have united many members of Congress in their concern. They have been skittish over his repeated suggestions that the nation had not meddled in the United States’ presidential election, contrary to copious intelligence information, and his frequent hints that his administration could lift sanctions against Russia. (On Thursday, the ambassador to the United Nations, Nikki R. Haley, said the United States would not lift the sanctions until Russia withdrew from Crimea.)