But senior Republican congressional aides said that outcome appeared unlikely, even if much of their party — and many Democrats — remained critical of the deal. Without explicit evidence of a breach by the Iranians, and given Europe’s support for the deal, there appeared to be little appetite among Republican leaders to pull the plug on Mr. Trump’s behalf.

“As flawed as the deal is, I believe we must now enforce the hell out of it,” Representative Ed Royce, the Republican chairman of the House Foreign Affairs Committee, said on Wednesday. He urged Mr. Trump to provide Congress with clear instructions for what he was trying to do.

“Whatever he decides, it is critical that the president lay out the facts,” Mr. Royce said. “He should explain what his decision means, and what it doesn’t.”

Mr. Royce and others in the House have argued instead for stepped-up enforcement and the enforcement of additional, targeted sanctions against Iran’s missile program and Hezbollah. Lawmakers were also preparing for the possibility that Mr. Trump could simply ask them to amend the law that requires him to recertify the deal every 90 days.

Any legislative outcome is likely to hinge on the Senate, where Democrats are confident they can hold most of their members together to preserve the deal. Of the four Senate Democrats who voted against the deal, two — Chuck Schumer of New York, the minority leader, and Ben Cardin of Maryland, the top Democrat on the Foreign Relations Committee — have indicated they would not move to abandon it.

That leaves two other previous Democratic no votes, Robert Menendez of New Jersey and Joe Manchin of West Virginia, undecided. But Republicans could also lose some of their own, like Senators John McCain and Jeff Flake of Arizona, Susan Collins of Maine or Rand Paul of Kentucky.