President Obama has put Senate Majority Leader Harry Reid in a bind.

Obama counts on the Senate’s top Democrat to help preserve the White House agenda on Capitol Hill. But on the issue of Iran, the president is facing a stampede of Democratic defiance, and Reid is the only one who can prevent an intraparty split.

Repeated veto threats from Obama have not curbed the appetite among a growing bipartisan swath of senators for a new Iran sanctions bill. Back in December, when the bill was only attracting the support of two dozen senators and had little chance of passing, Reid signaled he would bring the measure up for a vote.

Talks on an initial temporary nuclear deal between Iran and world powers, though, have made lawmakers even more skeptical of Tehran’s ability to negotiate in good faith.

Dozens of senators — including 16 Democrats — now support the new sanctions bill, drafted by Senate Foreign Relations Committee Chairman Bob Menendez, D-N.J., and Sen. Mark Kirk, R-Ill., bringing the total number of co-sponsors to 59.

Senate aides say far more senators – a veto-proof majority – would vote in favor of the bill if it reaches the floor. Sen. Chuck Schumer, D-N.Y., a top member of the Senate Democratic leadership and an aggressive advocate for Israel, is also leading the push to rally Democrats behind the measure.

The momentum now has Reid equivocating, with the Senate leader saying he will let the divisions “play out” before deciding whether to hold a vote on the bill.

The foreign policy stakes couldn’t be higher for Obama, who has vowed to veto any new sanctions while negotiations with Iran continue. The president is calling for more time to craft a permanent deal and has vowed to back sanctions if Iran fails to comply with the international community.

The United States is under increasing pressure from Israel and Western allies to prevent Iran from developing a nuclear weapon and has built a tough sanctions regime that has brought Tehran to the negotiating table.

When new Iranian President Hassan Rouhani signaled a willingness to engage, Obama seized on the opportunity. Secretary of State John Kerry offered a six-month preliminary deal that would require Iran to freeze some parts of its nuclear program and allow international inspectors access to nuclear sites in exchange for nearly $7 billion in sanctions relief.

But many prominent senators, deeply distrustful of Iran and not yet convinced that the Obama administration can exact lasting concessions, want to ratchet up pressure. Reid’s wavering on whether to hold a vote on the new sanctions bill has only served to intensify the public debate.

Opponents of the bill have begun to push back, with Sens. Jeff Merkley, D-Ore., and Chris Murphy, D-Conn., publicly urging their colleagues to hold off.

Sandy Berger, a former national security adviser to President Clinton, warned that a vote on the legislation raised the “risk of upending the negotiations before they start,” while former Sen. Dick Lugar, R-Ind., called on Congress “to give diplomacy a chance.”

The Menendez-Kirk bill would allow Obama a year of diplomacy before new, stronger sanctions would kick in, and supporters say they don’t see how that could disrupt negotiations with Iran.

Sen. Ben Cardin, D-Md., told the Washington Examiner that passage of the bill shouldn’t damage the deal with Iran in any way.

“The bill makes it clear that we prefer that [diplomatic] route, but if Iran doesn’t comply with that route, not only will sanctions be reimposed but they will be tougher sanctions,” he continued. “I think that’s consistent with the U.S. strategy.”

But some Democrats’ support may be only symbolic. One of the Iran bill’s co-sponsors, Sen. Richard Blumenthal, D-Conn., said this week he didn’t see the need for a vote “as long as there is progress” in implementing the initial nuclear agreement.

With so much bipartisan support for the bill, though, others say it’s time for Reid to hold the vote and test the Senate’s will on the issue.

The pressure on Reid will likely increase, making it harder for the Senate leader to stall his colleagues and protect Obama’s Iran talks.

“Partisanship rules everything here,” Kirk said. “But the Senate should be heard on this issue — this issue should not be decided by one senator.”