WASHINGTON — A letter to Iran from Senate Republicans aimed at undermining the Obama administration’s attempt to strike a diplomatic agreement to halt the country’s nuclear program appears to be backfiring.

Led by freshman Sen. Tom Cotton (R-AR), 47 Republicans used the letter to inform Iran’s leaders that such an agreement would be “nothing more than an executive agreement between President Obama and Ayatollah Khamenei.” They said the “next president could revoke such an executive agreement with the stroke of a pen and future Congresses could modify the terms of the agreement at any time.”

Conspicuously absent among signatories to the letter is Sen. Bob Corker (R-TN), the chairman of the Foreign Relations Committee, who says he’s working to build a veto-proof majority for his legislation restricting President Barack Obama’s negotiating options with Iran and ensuring congressional approval before any deal is struck. He hinted that the Cotton letter wouldn’t help advance the cause.

“I knew it was going to be only Republicans on [the letter]. I just don’t view that as where I need to be today,” Corker told Politico. “My goal is to get 67 or more people on something that will affect the outcome.”

Corker needs 13 Democrats to reach a veto-proof 67 votes, and the letter hasn’t earned him any favors. Senate Democrats are rallying to Obama’s side and attacking the Republicans for what they describe as an extraordinary act of openly undercutting a president during sensitive foreign policy negotiations.

Sen. Bill Nelson (D-FL), a Democratic cosponsor of Corker’s bill, said the letter “goes a step too far.”

“When we see a major party, on a partisan basis, try to undercut and kill the negotiations while they are going on at this very moment in Geneva, then that goes a step too far. I am saddened,” he said Tuesday on the Senate floor.

Obama accused the Republicans of “wanting to make common cause with the hardliners in Iran.” Vice President Joe Biden, for his part, issued a lengthy, blistering statement on Monday night saying the gambit was “beneath the dignity of an institution I revere” as it was “expressly designed to undercut a sitting President in the midst of sensitive international negotiations.”

Senate Minority Leader Harry Reid (D-NV) said on the floor Monday, “This is a hard slap in the face of not only the United States and the world. This is not a time to undermine our commander-in-chief purely out of spite.”

Iranian Foreign Minister Javad Zarif wasn’t moved by the letter, reportedly declaring that “in our view, this letter has no legal value and is mostly a propaganda ploy.” He lashed back at Cotton and his allies, pointing out technical errors in their description of how the U.S. Constitution works.

Meanwhile, Senate Republican leadership has been silent amidst the criticism. Senate Majority Leader Mitch McConnell (R-KY), who along with his leadership team signed the letter, did not mention it in his floor remarks on Tuesday morning.

A spokesman for House Speaker John Boehner (R-OH) wouldn’t comment on the letter, calling it “a Senate issue.”

The letter is drawing media criticism, too. The New York Daily News splashed the word “TRAITORS” under photos of Cotton, McConnell and presidential hopefuls Sens. Ted Cruz (R-TX) and Rand Paul (R-KY), who signed the letter.

The gambit is earning attention well outside traditional foreign policy circles. As of Tuesday morning, the hashtag #47Traitors was the No. 1 trending topic on Twitter in the United States.

Cotton is unfazed by the criticism. He stood by his letter in appearances Monday on CNN and Tuesday on MSNBC, saying that he wants Iran to dismantle its nuclear program “forever” — not for the 10 or 15 years that reportedly make up the duration of the deal that the Obama administration is closing in on.

“The point we’re making to Iran’s leaders, who, if you talk to many of the Iran experts, will say don’t understand our Constitution, is that if Congress doesn’t approve a deal, Congress won’t accept a deal,” Cotton said on MSNBC. “Now or in the future.”