WASHINGTON - Anger grew Tuesday over a letter from Senate Republicans to Iranian leaders designed to scuttle a yet-to-be-completed deal on its nuclear program, but Republican supporters stood their ground.

Congressional Democrats and independents, even some who question a deal with Iran, called the letter by freshman Sen. Tom Cotton (R., Ark.) an unprecedented act of political sabotage aimed at President Obama.

Democratic Sen. Tim Kaine of Virginia asked: "Is the Senate capable of tackling challenging national security questions in a mature and responsible way?"

Iranian Foreign Minister Javad Zarif dismissed the letter as "mostly a propaganda ploy" designed to undermine the talks between his country, the United States, and five other world powers.

The 47 Republican signers held firm Tuesday, saying that offering their opinion of a nuclear deal in a message to Tehran's leaders fulfilled part of their "advise and consent" role.

Cotton's letter gained more support Tuesday when Louisiana Gov. Bobby Jindal, a possible 2016 Republican presidential candidate, added his name to the letter. Former Texas Gov. Rick Perry tweeted that he "would be proud and honored to sign."

Former Florida Gov. Jeb Bush didn't say whether he'd sign the letter, but he blamed the administration for the situation, saying the senators "would not have been put in this position had the administration consulted regularly with them rather than ignoring their input."

Wisconsin Gov. Scott Walker also did not say that he'd sign the letter, but said, "Unless the White House is prepared to submit the Iran deal it negotiates for congressional approval, the next president should not be bound [by] it."

The support was not unanimous.

Sen. Bob Corker of Tennessee, chairman of the Senate Foreign Relations Committee, was one of seven Senate Republicans who refused to sign. He said Tuesday he "didn't think it was going to further our efforts to get a place where Congress would play the appropriate role that it should in the Iran negotiations."

Cotton also sharply criticized Vice President Biden, a former senator, who called the letter "beneath the dignity of an institution I revere" in a scathing statement late Monday evening.

Biden "has been wrong about nearly every major foreign-policy and national security decision in the last 40 years," Cotton said Tuesday on MSNBC's Morning Joe. "If Joe Biden so respects the dignity of the institution of the Senate, he should be insisting that the president submit any deal to approval of the Senate."

In his letter, Cotton warned that a nuclear deal probably wouldn't survive beyond Obama's presidency.

The Obama administration continued to vent its anger Tuesday. White House deputy press secretary Eric Schultz called the letter "a flagrant, partisan attempt to interfere with the negotiations."

Former Secretary of State Hillary Rodham Clinton took time in her news conference defending her use of personal e-mail during her tenure to denounce the letter. "Either these senators were trying to be helpful to the Iranians or harmful to the commander in chief in the midst of high-stakes international diplomacy," she said.

A debate raged off Capitol Hill as well.

Danielle Pletka, of the conservative American Enterprise Institute, said, "Whenever you try to exclude lawmakers the way the Obama administration has in a very determined fashion, you lose the buy-in, that's just the reality."

Anthony Cordesman, with the Center for Strategic and International Studies, said the letter was "an appalling mess."

"One thing about sending international letters, they better be serious, they better be well-drafted, they better have a convincing intellectual content, and they better not seem just a hollow political gesture for partisan purposes," he said. "This letter can't meet any of those four tests."