Republicans in control of Congress reacted with a collective shrug on Tuesday to the revelation that top Trump administration national security officials had used a messaging app to chat about military operations, even as Democrats expressed outrage and called for an investigation and resignations.

One day after the existence of a group chat about airstrikes in Yemen — and the inclusion of a journalist in it — came to light, leading Republicans and most rank-and-file G.O.P. lawmakers refrained from criticizing the administration, though some suggested they might look into how the breach had occurred.

Senator John Thune of South Dakota, the majority leader, told reporters that there were “errors in judgment” among the officials who took part in an encrypted group chat on the app Signal. He initially suggested it was up to the Trump administration to address them, but later appeared open to the possibility of the relevant Senate committees beginning their own inquiry.

“I suspect the Armed Services Committee may want to have some folks testify and have some of those questions answered,” he said during a news conference on Tuesday afternoon. “What we want to do is make sure that something like that doesn’t happen again.”

Speaker Mike Johnson read directly from a White House press statement during a news conference on Capitol Hill, and rejected the notion of firing Michael Waltz, the national security adviser, who had set up the group chat and included Jeffrey Goldberg, the editor in chief of The Atlantic.

“He was born for the job. He is highly qualified,” Mr. Johnson told reporters. “The president said he has total confidence in him, and we do as well.”

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