– If the negotiations with Iran produce the framework of a deal, President Obama will face a leviathan task of selling it to a skeptical Congress.

If they fail to produce anything by a self-imposed Tuesday deadline, he’ll confront an equally daunting challenge of holding off congressional moves to impose new sanctions on Iran while he tries to buys more time for negotiations.

In either case, Obama will have to make the case for what would be a crowning foreign policy achievement to a public that distrusts Iran’s regime, yet doesn’t want an unresolved nuclear crisis to spark a nuclear arms race, or even a war, in the Middle East.

“There is no trust in Iran,” said Sen. Ben Cardin, a Maryland Democrat on the Foreign Relations Committee. “There is not a real belief that they want to come to a real deal.”

Striking an agreement that prevents Iran from developing a nuclear weapon would be a capstone for Obama’s foreign policy in his second term and vindicate his pursuit of diplomatic engagement over military action, one of the central arguments of his campaign for the presidency.

Any provisional accord reached by Tuesday night between Iran and six world powers — China, France, Germany, Russia, Britain and the United States — would only clear the way for the most difficult negotiations. Three more months of talks would follow in an effort to reach a detailed, technical agreement by June 30. The two sides will wrangle over every word, line and comma, according to U.S. officials involved in the talks at the Swiss lakeside resort of Lausanne.

Even if Tuesday’s deadline passes without a framework agreement, it’s possible that negotiations might continue until June 30, when the so-called Joint Plan of Action restricting Iran’s nuclear program is set to expire.

Lawmakers are poised to step in in either case. Congressional Democrats as well as Republicans are lining up against the administration and behind legislation that would allow them to review and possibly overturn any deal that Secretary of State John Kerry brings home. If there’s no deal, another measure would hit Iran with new sanctions.

“The administration is in a tough spot right now,” said Suzanne Maloney, a former State Department staff member who is an Iran analyst at the Brookings Institution in Washington. The proposals being floated in Congress “would wreck any deal and undercut the multilateral diplomacy that has brought the process this far.”

Some of the blame lies with the White House for not fully engaging with Congress or clearly defining an Iran policy, Maloney said.

Politics is at play, as well. For Republicans, “imploding what would be a major — perhaps the major — foreign policy achievement for the administration represents too tempting of a target,” Maloney said.

White House spokesman Eric Schultz said the administration has briefed and consulted with Congress hundreds of times as the negotiations with Iran have progressed. “That includes one-on-one meetings, phone calls, briefing sessions, both classified and unclassified, with a range of officials, including the president,” he said.

Schultz repeated that Obama would veto the main Iran legislation being debated, sponsored by Sens. Bob Corker, R-Tenn., and Robert Menendez, D-N.J.

That bill would require the administration to wait 60 days before suspending any sanctions against Iran, during which time lawmakers would have the opportunity to approve, reject, or take no action on the deal.

The Senate voted 100-0 last week to increase sanctions on Iran if the president can’t certify that Iran is complying with a nuclear deal. The budget amendment is symbolic, and doesn’t carry the force of law because it was attached to a budget resolution. It was co-sponsored by Sens. Mark Kirk, R-Ill., and Sherrod Brown, D-Ohio.

A tougher measure, sponsored by Kirk and Menendez, would increase sanctions on Iran if there isn’t a deal curtailing its nuclear program by the end of June.

“If they come back without a deal, they’re going to get hit with the Kirk-Menendez bill. If they come back with a deal, they’re going to get hit with Corker’s bill,” said Omri Ceren, press and strategy director at the Israel Project, a Washington-based group that describes itself as educating the public about issues affecting Israel and the Middle East.

Strong opposition to the Iranian regime and concern over its nuclear program is perhaps the one foreign policy issue that has remained bipartisan on Capitol Hill in the last five years since Congress began to ratchet up sanctions in 2010.