“It was kind of a very rapid process. Everybody was looking forward to getting out of town because of the snowstorm,” McCain said. “I think we probably should have had more discussion about it, given the blowback that there is.”

On this at least, Democrats and Republicans found agreement.

“I find it hard to believe that they understood the severity of what they were doing,” said Sen. Debbie Stabenow (D-Mich.).

Though none of the 47 Republican signers has expressed regret for co-signing it, the missive, authored by freshman Sen. Tom Cotton, is creating unexpected fallout in Congress. And it threatens to linger politically and legislatively.

Sensing a public relations advantage, the campaign arm for Senate Democrats on Wednesday quickly circulated newspaper op-eds criticizing Republicans who signed the missive, and strategists said the issue will soon show up in TV ads in states of vulnerable senators. Democratic leader Harry Reid (D-Nev.) took to the Senate floor for a second time to blast Republican “gimmicks” on Iran and Secretary of State John Kerrycalled it a “stunning” breach of protocol after being teed up by a question from a Democratic senator at a committee hearing.

On the legislative front, a fragile bipartisan coalition of Iran hawks, who had been approaching a veto-proof majority for legislation that could potentially scuttle any U.S.-Iran nuclear agreement, was showing signs of cracking, as some centrist senators warned they were close to backing away from the measure.

Sen. Angus King of Maine, an independent who caucuses with the Democrats, said in an interview he currently backs the legislation designed to give the Senate more input on a nuclear deal, but he cautioned that the Iran letter is making him think twice.

“If I’m not convinced that this issue can be handled on the merits and not on a partisan basis,” he said, “then I’m going to change my mind.”

The Democratic Senatorial Campaign Committee wasted little time in using negative coverage of the letter as grist for the campaign trail. Throughout the day officials took aim at vulnerable incumbents by blasting out a series of newspaper editorials including one from the Pittsburgh Post-Gazette, which wrote that Sen. Pat Toomey (R-Pa.) and his colleagues should be “ashamed” for signing the letter.

In an interview, Toomey said he “didn’t have any particular anticipation of the level of controversy” surrounding the letter. Asked whether Democratic attacks on his support for the missive will be effective, Toomey responded: “Uh, no.”

“They’ll use whatever they think works,” he said. “That letter is just the most recent case of my doing all that I can to prevent Iran from having a nuclear bomb.”

Ohio’s Democratic former governor and current Senate candidate, Ted Strickland, lit into GOP incumbent Sen. Rob Portman for his “reckless” support of the letter. In an interview, Portman said he wasn’t even aware of the attack from Strickland or of Democrats’ circulation of a Cleveland Plain Dealer editorial that bashed the “usually rational” senator.

Instead, Portman said Democrats, especially those in the White House, are viewing the letter all wrong.

“We need a verifiable, strong agreement with Iran. And I think the letter helps us get there,” Portman said. “The Iranians are tough negotiators. [The White House] shouldn’t view it politically. They should use it to try to get a better agreement.”

But there appeared to be little hope of turning back the clock on the politicization elicited by the letter, and multiple Democratic strategists predicted there would be more political repercussions to come for Republicans. One said there’s “no question” that Democrats will run ads attacking Republicans who signed onto the letter.

Republicans say they are ready for it. In an interview, National Republican Senatorial Committee Chairman Roger Wicker of Mississippi said Democratic attacks on the Iran issue will be no more effective than the dissipating “wailing and gnashing of teeth” over Israeli Prime Minister Benjamin Netanyahu’s invitation by House Republican leaders to address a joint a meeting of Congress.

“They do so at their peril. I think they really are dancing on a pin there,” Wicker said.

The Cotton letter elevated an already tense atmosphere in the Capitol as the Iranian negotiations entered their final weeks.

Though Cotton has insisted that Democratic senators were approached about the letter, neither Bob Casey of Pennsylvania nor hawks like Richard Blumenthal of Connecticut said they had been approached. POLITICO was unable to locate a Democrat who was aware of the letter before it became public.

“I’d like to know what Democrat he approached,” said Sen. Claire McCaskill (D-Mo.).

In January, Democrats informed Obama they would not vote for the sanctions bill on the Senate floor until after the March 24 deadline, and last week they set a similar deadline on legislation from Sen. Bob Corker (R-Tenn.) that would let Congress weigh in on an Iran deal.

Casey said he still supports an Iran sanctions bill but bemoaned that a letter was sent with such a partisan tilt.

“It’s not helpful, I can say that for sure,” Casey said. “Everything that has been done with the Iranian nuclear issue for years has been bipartisan. Why would people go in the direction of not having it bipartisan?”

Corker abstained from signing the letter over fears it would hurt his drive toward a veto-proof majority in support of his legislation to let Congress weigh in on an Iran deal, and Democrats lauded his stance. But his colleagues’ signatures still caused damage among the 10 Democrats who have expressed support for his measure.

“Right now, I’m in. But I’m waiting to see further developments,” said King. “The irony here is I believe we were headed toward a very substantial majority if not a veto-proof majority.”

Manu Raju contributed to this report.