In a press conference Tuesday afternoon, Hillary Clinton absolutely slammed the controversial letter to Iran organized by Senator Tom Cotton and signed by 47 Republicans. "Either the senators were trying to be helpful to the Iranians or harmful to the commander in chief," Clinton said. "The recent letter from Republican senators was out of step with the best traditions of American leadership."

Clinton, the overwhelming favorite to win the Democratic nomination for president in 2016, is no one's idea of an Iran dove. That said, it's still not super-surprising that she's lining up behind Obama on this issue. The letter is explicitly designed to undermine Obama's attempt to reach a nuclear agreement with Iran.

This is a direct strike at the president's ability to conduct foreign policy without congressional interference. Presumably, Clinton doesn't like the precedent: after all, Senate Republicans could do something similar to a President Clinton in 2017.

This also shows how Republicans are succeeding in making Obama's Iran negotiations a partisan issue. Senate Democrats are lining up behind Obama to bash the GOP letter. "I never would have sent a letter to Saddam Hussein," Sen. Debbie Stabenow said, in a voice the New York Times describes as "shaking with rage." Clinton wouldn't want to side with Republicans against the Democrats just as campaign season is about to begin in earnest.

It's exactly this growing partisanship that may cause Cotton's letter to backfire. To blow up the talks by passing new sanctions on Iran, Senate Republicans need Democratic votes to overcome Obama's veto. But if the Iran negotiations become not just an Obama position, but a hard-line partisan one, then Democrats will be more likely to vote with the administration.

Here's the full text of Clinton's Iran remarks at the presser, courtesy of the Washington Post: