Republicans in Congress have been militant in backing President Trump during the impeachment investigation. But after Mick Mulvaney , the acting White House chief of staff, appeared to admit to a quid pro quo with the president of Ukraine on Thursday, that support showed signs of cracking.

“You don’t hold up foreign aid that we had previously appropriated for a political initiative. Period,” Senator Lisa Murkowski said. Representative Adam Kinzinger told CNN that it was “quite concerning.”

An impeachment interview planned for Laura Cooper, a senior official at the Defense Department who specializes in Russia, Ukraine and Eurasia, was postponed until next week.

A rare Republican critic

As Congress left for the week, a few Republicans dared to criticize Mr. Mulvaney. I spoke with one of them: Representative Francis Rooney of Florida, a former ambassador who sits on the House Foreign Affairs Committee.

What was your reaction to what Mick Mulvaney said?

I thought, “Wow, O.K.” We’ve had this president saying that the withholding of the funds was not related to his pressure of the new president of Ukraine. I gave the guy the benefit of the doubt. And then this week we had some testimony from the impeachment hearings. Then this thing comes out from Mulvaney. It looks like game, set, match to me on that issue. I heard the words like anyone else, and I couldn’t believe it.

What did you think of Mr. Mulvaney’s attempt to revise what he said?

I don’t know how you can walk it back. It’s not like, “I said these three vague things and one thing was misinterpreted.” I didn’t see a lot of gray area in what he said. It was pretty clear.

Why did you react the way you did?

There are a few things that really bother you. I’ve been involved in the diplomatic business. I have great respect for our professional diplomats who go about their work. This has undermined them and put them in a bad light. Like Senator Murkowski said, I don’t think it’s right to use the power and prestige of the U.S. for a political situation. Our foreign policy is supposed to advance the interests of the United States. That’s what we’re in the business of.