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

"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 e-mail late Monday. "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 that 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.

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

"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.