WASHINGTON – Sen. Pat Toomey (R., Pa.) added his signature to a hotly-debated  letter to Iranian leaders warning them that any nuclear deal they strike with President Obama might not last.

The letter, signed by 47 Republicans, is aimed at cutting off a potentially bad deal that would leave Iran with a route to obtaining nuclear weapons, according to those who signed it.

It has provoked a scathing response from Democrats – who say it undermines the office of the presidency and the White House's power to set foreign policy, and undercuts negotiations aimed at avoiding a military conflict. The letter has also brought more subtle critiques from some key Republicans who say it could hurt the bipartisan cooperation needed for the Senate to truly influence the outcome in Iran, the most pressing foreign policy issue of the moment.

"This letter is an extension of Sen. Toomey's long and active record of engagement in the effort to prevent Iran from obtaining nuclear weapons," a Toomey spokeswoman wrote in an email. "It's critical that the Iranians know that a bad deal that fails to win congressional approval is a deal that will not stand for long."

The letter says the next president could reverse any deal "with the stroke of a pen" and that a future Congress could also change the terms.

Toomey, known mainly for his conservative stands on fiscal policy, staked out a tough position on Iran in February, calling for tougher sanctions and a Senate vote on any agreement on Tehran's nuclear program.

This week,

, said the letter “ignores two centuries of precedent and threatens to undermine the ability of any future American president, whether Democrat or Republican, to negotiate with other nations on behalf of the United States.”

He added, "Honorable people can disagree over policy. But this is no way to make America safer or stronger."

The White House has warned that disrupting negotiations will leave military action as the only option to prevent Iran from obtaining nuclear weapons. Many Democrats have said lawmakers should wait for the details of a final deal before judging it, and White House officials have said that they would prefer no deal to a bad one.

Potential Toomey challenger Joe Sestak, a former admiral and Delaware County congressman, seized on the controversy to play up his foreign policy credentials.

"Senators have the absolute right to argue and disagree with the president's approach to any issue. But for Sen. Toomey to sign a letter to a foreign leader urging that leader to ignore the institution of the American presidency is inexcusable, embarrassing, and shows a lack of experience and understanding about America's standing in the world, led first and foremost by the U.S. President," Sestak, a Democrat, said in a news release.

Sestak's release opened by citing his 31 years in the Navy and past job as director for defense policy in the Clinton administration.

Sen. Bob Menendez (D., N.J.), the top Democrat on the Senate foreign relations committee and a vocal critic of Obama's Iran poilcy, said, "As far as I'm concerned you saw that it was a partisan letter. Tells you everything about it."

The senator behind the letter, Tom Cotton (R., Ark.), who served with the Army in Iraq and Afghanistan, fired back at critics, saying the signatories were "simply speaking for the American people."

The point, he told MSNBC's Morning Joe, "is that if Congress doesn't approve a deal, Congress won't accept a deal. Now or in the future."

Former Pennsylvania Republican Sen. Rick Santorum said Tuesday that he is "grateful" that the Senate "is exercising their constitutional prerogative to stop this reckless diplomacy" by Obama.

But other Republicans worried that the tactic might break down the bipartisan cooperation needed to impose their will on any deal and force through new sanctions if the negotiations fall apart. Sen. Bob Corker (R., Tenn)., chairman of the foreign relations committee, is pushing for a bill to give the Senate a formal vote on any nuclear agreement with Iran, and Menendez has teamed with Republicans on new sanctions. They have been working to gather enough Democratic votes to override a potential veto of each measure. Corker suggested that a letter signed only by Republicans could undermine cooperation.

"My goal is to get 67 or more people on something that will affect the outcome," Corker told Politico.

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