The most important person in the U.S.-Iran nuclear negotiations right now may be Harry Reid, the Senate Democratic leader from Searchlight, Nev.
Two weeks ago, President Obama's nuclear diplomacy was in trouble, but not because of anything Iran was doing. The problems were domestic.
A Senate bill proposing new economic sanctions against Tehran had swiftly gathered 59 supporters, a solid majority and only one vote short of the number needed to prevent a filibuster. The bill's backers, including the American Israel Public Affairs Committee, the powerful pro-Israel lobbying organization, were pressing for a quick vote in the Senate.
The sanctions wouldn't hit Iran immediately — or possibly ever. They would be triggered in the event of a breakdown in the nuclear negotiations, a missile test or even a terrorist attack by Iran's Lebanese ally, Hezbollah.
The problem is that Iran has said it would consider passage of the bill as a violation of the agreement the U.S. signed in November, which promised no new sanctions while negotiations were underway. Obama aides have warned that if the bill passed, Iran might walk out of the talks, and that it would have the sympathy of much of the rest of the world if it did.
But the bill's authors were willing to live with the risk. They aren't happy with the terms Obama agreed to with Iran, and they say their aim is to strengthen the president's hand. It's hard, however, to see the proposal as anything but a direct rebuke to Obama over his conduct of foreign policy.
Initially, the bill had impressive momentum, with 16 Democrats joining 43 Republicans in support. Its backers predicted that they would soon have more than 60 votes, the number needed to move a bill forward in the Senate.
But then Reid planted his feet. He controls the Senate calendar, and he let senators know that he saw no need to act on the sanctions bill soon. "At this stage, I think we're where we should be," Reid blandly told reporters.
That strategic inaction gave Obama and White House aides more time for some furious personal lobbying against the bill. And it gave a coalition of liberal groups time to organize a grass-roots lobbying effort aimed at countering AIPAC's appeals to Democratic senators who hadn't yet decided.
By last week, it was clear that Reid had prevailed; the Senate isn't likely to vote on new sanctions any time soon.
The number of cosponsors has stalled at 59 senators, and a few of the Democrats who had signed on were beginning to edge away: Sen. Richard Blumenthal (D-Conn.), one of the cosponsors, said he didn't think the Senate needed to move "as long as there's progress" in the negotiations.
Another, Sen. Michael Bennet (D-Colo.), feels much the same way, a spokesman told me. As a result, pressure is waning on Reid to allow a vote on the bill.
But this was just one round in the ongoing Washington battle over how to deal with Iran, and the closer the nuclear negotiations bring us to a final agreement, the more intense the debate will become. That makes it worth examining this early dust-up for clues to how later battles will shape up.
One thing worth noting is that the strongest support for Israel's hard line these days comes from Republicans, not Democrats. Nearly three-quarters of the sanction bill's sponsors were Republicans, and, as the Jewish Daily Forward noted puckishly last week, most of the Senate's 10 Jewish members, all of whom are Democrats, didn't back it.
Support for Israel in Congress has been bipartisan for decades, of course. But the hawkish policies of Israeli Prime Minister Benjamin Netanyahu have put off many Democrats, and they are stopping far short of what Israel's most ardent supporters are asking.
Democrats also have more incentive to back the president on a core issue of foreign policy, of course, since he's from their party.
More fundamentally, though, Democrats tend to agree with Obama on the merits of the issue: that the negotiations are the last, best chance to persuade Iran to limit its nuclear program, and that disrupting the talks could make a military conflict inevitable.
"In my view, it is a march toward war," Sen. Dianne Feinstein (D-Calif.) said of the sanctions bill.
That doesn't mean there hasn't been fallout for traditionally pro-Israel Democrats. Senate aides say their bosses have been barraged by appeals from voters and donors, many apparently mobilized by AIPAC, asking them to support the sanctions bill. But there has also been a wave of messages from another traditional Democratic constituency: antiwar voters and donors mobilized by organizations including MoveOn.org and J Street, a liberal pro-Israel group that casts itself as a dovish alternative to AIPAC.
That battle for hearts and minds is certain to continue, especially if a deal is reached that would require Congress to unwind years of sanctions legislation.
And if a final agreement is reached on schedule, roughly six months from now, it will land in the middle of the fall campaign for the Senate, timing that is certain to heighten the rhetoric.
But even before then, if Obama manages to move his nuclear diplomacy forward, he'll have Harry Reid to thank.