Washington (CNN)Hillary Clinton lambasted Senate Republicans who penned a letter aimed at undermining President Barack Obama's nuclear negotiations with Iran, calling the move "out of step with the best traditions of American leadership."
Hillary Clinton blasts Cotton Iran letter
The Democratic frontrunner for the 2016 presidential campaign told reporters at the United Nations on Tuesday that "one has to ask, what was the purpose of this letter?"
"There appear to be two logical answers," Clinton said. "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. Either answer does discredit to the letter's signatories."
Her comments come after 47 Senate Republicans signed a controversial letter to Iran warning the country's leaders that a lasting nuclear deal would have to be approved by Congress.
Sen. Tom Cotton, who spearheaded the letter, said Tuesday he'd "welcome" presidential candidates to join the letter's signatories. And Cotton said he would welcome "even Hillary Clinton," the presumptive Democratic presidential frontrunner, to join the effort.
"I suspect she might have reservations about this ill-fated nuclear deal with Iran as well," Cotton said on CNN's New Day.
Several potential GOP presidential candidates have already signed onto the letter. Sens. Ted Cruz, Marco Rubio, Rand Paul and Lindsey Graham are among the 47 Republicans who signed onto Cotton's letter "to the Leaders of the Islamic Republic of Iran."
Louisiana Gov. Bobby Jindal, a potential 2016 contender, signed the letter Tuesday, according to his spokeswoman.
And former Texas Gov. Rick Perry would "absolutely" sign it, his spokesman said, adding that "if President Obama signs an agreement that the Congress cannot support, our next president should not be bound by it."
Many of the 47 Republican senators who'd signed the letter defended it on Tuesday. Cruz said he wishes Obama's administration "showed even a fraction of the seriousness and backbone toward Iran that they should manifest in terms of hostility to Congress."
"The Constitution requires that any treaty be submitted to the Senate for ratification," Cruz said. "Unfortunately President Obama has repeatedly defied the law and defied the Constitution. And this Iran deal I believe is a historic mistake. I believe it endangers the national security of this country."
Asked if he regrets signing the letter, Rubio said: "Regret? I would send another one tomorrow."
"The risk of a nuclear Iran is so great that we had to do everything possible to keep us from finding ourselves in a situation where we're going to have a nuclear Iran," Rubio said.
But seven GOP senators declined to sign it, hinting that signing on to the letter could be counterproductive.
Sens. Jeff Flake, Lisa Murkowski, Lamar Alexander, Dan Coats, Thad Cochran, Susan Collins and Bob Corker didn't sign the letter. Murkowski and Coats are the only two up for re-election, and Coats is rumored to be considering retirement.
Flake's spokeswoman Bronwyn Lance Chester said the senator agreed with the spirit of the letter, but abstained from signing it because he did not "believe the letter was necessary."
Corker, who's chairman of the Senate Foreign Relations Committee and has been pushing a bill to require congressional review of any deal, indicated that measure was his most pressing concern.
"As chairman of the Senate Foreign Relations Committee, Sen. Corker's focus is on getting a veto-proof majority to support his bipartisan bill for congressional review of any comprehensive nuclear agreement with Iran," an aide said.
Collins said while she agrees any Iran agreement should come before the Senate, this letter won't likely sway negotiations.
"I don't think that he Ayatollah is going to be particularly convinced by a letter from members of the Senate even when signed by a number of my distinguished and high-ranking colleagues," she said on Monday.
President Barack Obama slammed the letter on Monday, accusing the GOP of making "common cause with the hard-liners in Iran" by attempting to undercut ongoing negotiations that face a first deadline for a framework agreement at the end of the month.
Critics have accused the Senate Republican signatories of attempting to blow up delicate talks and undermining Obama's authority in an unprecedented show of partisanship on the international stage.
But Cotton said Tuesday on CNN's "New Day" he doesn't believe the letter is in any way "unprecedented."
Instead, Cotton countered that Obama's attempts to reach a nuclear accord with Iran without congressional approval would be unprecedented.
Senate Republicans, supported by a number of Democrats including Robert Menendez, introduced legislation that would give Congress a chance to weigh in on a nuclear deal. That bill's Republican sponsor, Foreign Relations Committee Chairman Sen. Bob Corker, did not sign onto Cotton's letter.
But even if that bill falters, Congress would have to pass legislation to lift economic sanctions on Iran to allow a nuclear deal to go into effect -- giving lawmakers an opportunity to weigh in then.
In the face of mounting criticism in the U.S. and abroad -- including from Vice President Joe Biden, a Senate veteran and enthusiast -- Cotton insisted that the letter was not intended as a partisan move and was instead aimed at keeping the U.S. from reaching a bad and "dangerous" deal.
"This letter is about stopping Iran from getting a nuclear deal," Cotton said. "One way that we make sure that we get a better deal is that we stand strong."
"We're not talking about just stopping Iran from getting a bomb today or tomorrow, but 15 years from now," Cotton added.