The danger should be obvious. That’s why mere expressions of outrage simply are no longer adequate.

It is precisely moments like this that the author of Federalist 51, generally thought to be James Madison, had in mind when he explained our system of checks and balances: “Ambition must be made to counteract ambition. The interest of the man must be connected with the constitutional rights of the place. It may be a reflection on human nature, that such devices should be necessary to control the abuses of government.”

Let’s be honest: Most Republicans have been too afraid of the president’s tweets to rise to the occasion. Their reluctance is understandable: In one poll, 79 percent of Republican voters said they were just fine with Mr. Trump’s performance in Helsinki. Breaking too decisively with the president risks offending the base and perhaps inviting a primary challenge. The end of the political careers of Senators Jeff Flake and Bob Corker and the defeat of Representative Mark Sanford are cautionary tales for many Republicans.

So Republicans who privately bemoan the president’s recklessness are content to rationalize, wring their hands and do nothing. But now they need to look past his tweets and even their own electoral base to the larger question of their constitutional responsibilities. Republican members of Congress need to act like a political party with principles rather than outsourcing their consciences to a handful of critics who are willing to say out loud what many of them are saying in private.

Taking action now is not merely a matter of recovering political courage and integrity; it would also be a fundamental reassertion of Congress’s Article I powers and our system of checks and balances. It also might save the political reputations of members of Congress who risk becoming footnotes for their complicity in Mr. Trump’s march of folly.

“The dam is finally breaking. Thankfully,” tweeted Senator Corker, who heads the Senate Foreign Relations Committee. But it will break only if he and his colleagues actually do something.