After helping to ignite a firestorm over a possible nuclear agreement with Iran, Senator John McCain, a former Republican presidential candidate, is now sort of acknowledging his error. “Maybe that wasn’t exactly the best way to do that,” he said on Fox News on Tuesday.

He was referring to the disgraceful and irresponsible letter that he and 46 Senate colleagues sent to Iran’s leaders this week that generated outrage from Democrats and even some conservatives.

The letter was an attempt to scare the Iranians from making a deal that would limit their nuclear program for at least a decade by issuing a warning that the next president could simply reverse any agreement. It was a blatant, dangerous effort to undercut the president on a grave national security issue by communicating directly with a foreign government.

Maybe Mr. McCain, who is chairman of the Senate Armed Services Committee, should have thought about the consequences before he signed the letter, which was drafted by Tom Cotton, a Republican of Arkansas, a junior senator with no foreign policy credentials. Instead of trying to be leaders and statesmen, the Republicans in Congress seem to think their role is outside the American government, divorced from constitutional principles, tradition and the security interests of the American people.

The letter was the latest shot to blow up the negotiations with Iran. Earlier this month, House Republicans invited Prime Minister Benjamin Netanyahu of Israel to denounce a pact in a speech to Congress, and a group of senators is pushing legislation that could set new conditions on a deal and force a congressional vote.

Besides being willing to sabotage any deal with Iran (before they know the final details), these Republicans are perfectly willing to diminish America’s standing as a global power capable of crafting international commitments and adhering to them.

Vice President Joseph Biden Jr. was blistering in his condemnation, saying, “This letter sends a highly misleading signal to friend and foe alike that our commander in chief cannot deliver on America’s commitments — a message that is as false as it is dangerous.” But perhaps President Obama described this bizarre reality best. “It’s somewhat ironic to see some members of Congress wanting to make common cause with the hard-liners in Iran,” he said. “It’s an unusual coalition.”

So far, the Iranians have largely dismissed the bumbling threat, with their foreign minister, Mohammad Javad Zarif, describing the letter as “propaganda.” But there are fears it could embolden hard-liners in Iran who, like the Republicans and some of the Democrats in Congress, oppose any nuclear agreement between Iran, the United States and its major allies.

The Republican efforts have so infuriated Democrats that even those who might have supported legislation that would have given Congress leverage over an Iranian pact are having second thoughts. Before this, the thinking was that the two bills most in play — one that would increase sanctions on Iran and another that would force the administration to bring any agreement to Congress for a review — might draw enough Democratic support to override a veto by President Obama. Both measures would surely scuttle a deal, but the Republicans’ actions may have set back their senseless cause.

The best and only practical way to restrain Iran from developing a bomb is through negotiating a strict agreement with tough monitoring. In rejecting diplomacy, the Republicans make an Iranian bomb and military conflict more likely.