Seth Wenig, Associated Press
Hillary Rodham Clinton speaks to the reporters at United Nations headquarters, Tuesday, March 10, 2015. Clinton conceded that she should have used a government email to conduct business as secretary of state, saying her decision was simply a matter of "convenience."

WASHINGTON — Hillary Rodham Clinton on Wednesday sought to turn a letter from Senate Republicans about nuclear negotiations with Iran into a litmus test in the 2016 campaign, saying that no one hoping to become the nation's commander in chief should sign it.

Clinton castigated the Republican letter to Iranian leaders for the second time in as many days. Her first comments on it during a news conference at the United Nations were overshadowed by questions about her use of a private email account while she served as secretary of state.

"GOP letter to Iranian clerics undermines American leadership," Clinton wrote in a message posted on her Twitter account. "No one considering running for commander in chief should be signing on."

The letter to Iranian leaders warned that unless Congress approved it, any nuclear deal they cut with President Barack Obama could expire once he leaves office. The letter could serve as a source of division in the upcoming presidential campaign: Democrats have assailed it as an attempt to undercut Obama at the negotiating table while Republicans have warned that a deal could lead to a nuclear-armed Iran.

The GOP letter was signed by 47 Republican senators, including three potential presidential candidates: Rand Paul of Kentucky, Marco Rubio of Florida and Ted Cruz of Texas.

Louisiana Gov. Bobby Jindal, another possible 2016 GOP candidate, responded to Clinton on Twitter: "No one who allows Iran to become a nuclear power should consider running."

Former Florida Gov. Jeb Bush and Wisconsin Gov. Scott Walker, meanwhile, issued statements on Tuesday suggesting their Senate colleagues were justified in sending the letter.

Bush said the senators were "reacting to reports of a bad deal that will likely enable Iran to become a nuclear state over time." Said Walker, "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 U.S. and five other nations have been working to reach an agreement that would bar Iran from being able to develop nuclear weapons. Tehran has said its nuclear program is peaceful.

But Republicans worry that Iran is not negotiating in good faith and any deal would be difficult to enforce and enable Iran to eventually become a nuclear-armed state.

Clinton said Tuesday the letter was "out of step" with American leadership and was an attempt either to be "helpful to the Iranians or harmful to the commander in chief" amid the negotiations. "Either answer does discredit the letters' signatories," she said.

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