President Barack Obama now has a framework nuclear deal with America's Iranian foe, but as he tries to build it into a binding accord he faces enemies closer to home.

Can he prevent the US Congress from torpedoing his diplomatic efforts to contain Tehran?

"If Congress kills this deal, not based on expert analysis and without offering any reasonable alternative, then it's the United States that will be blamed for failure of diplomacy," Obama said Thursday.

"International unity will collapse and the path to conflict will widen."

The president has not denied Congress' role in the process. His aides regularly troop to Capitol Hill to inform and consult members of the Republican-controlled legislature.

His chiefs of diplomacy, defense, the Treasury, and the military have all appeared before the Senate and the House of Representatives to address lawmakers' qualms over the negotiations.

On Thursday, the outlines of a final deal were unveiled in Switzerland after negotiations between six world powers and Iran. A final deal is to be completed by June 30.

But US lawmakers have proven tough to win over and still have deep reservations -- some real, some perhaps politically opportunistic.

Opponents of a deal believe Obama is obsessed with his place in history, and has made too many concessions, abandoning US allies Israel and Saudi Arabia in the process.

- Raising the bar -

Their eyes fixed on the 2016 elections, Republicans have sought to impose their own criteria on the deal, raising a bar that by design is higher than the commander-in-chief's: the halting of all uranium enrichment by Iran, transparence of its military programs, and closure of its underground facility at Fordo.

Thanks to the efforts of the Democratic minority in Congress, the administration has so far succeeded in using parliamentary procedures to delay two bills on Iran.

The furthest advanced of the two is the Corker-Menendez bill, sponsored by the Republican chairman of the Senate Foreign Relations Committee, Bob Corker, and Democrat Robert Menendez.

It would give the Congress the right to review any final agreement, before any US sanctions on Iran are lifted.

Obama has vowed to veto the bill in its current form, as well as a separate bill that would impose additional sanctions on Iran.

"We think it's best for members of Congress to take a look at the framework and then give the space to negotiate the final details between now and June," said a senior administration official.

Obama has exhorted lawmakers to rise above political gamesmanship, while warning that interfering in the negotiations increases the risk of a military confrontation.

- Democratic defections -

But the Republicans have not been deterred.

A committee vote on Corker-Menendez is still scheduled for April 14, when the Congress returns from a Spring break.

No date has been set for a vote by the Senate as a whole, however, and Senate Majority Leader Mitch McConnell, who is traveling abroad, has kept his silence.

"Rather than bypass Congress and head straight to the UN Security Council as planned, the administration first should seek the input of the American people," said Corker.

His Republican colleagues are following the same line. The White House's problem is that more and more Democrats are joining them.

Nine of 46 Democratic Senators are co-sponsors of Corker-Menendez, and only four more Democratic defections are needed for a veto-proof, two-thirds majority.

So a lot rides on the White House's ability to persuade skeptical Democrats to hold their fire until the end of June.

Questioned by AFP, some refrained from comment on Friday. Others, like the new ranking Democrat on the Senate Foreign Relations Committee, Ben Cardin, said they were taking their time to think it over. Still others, like Menendez and Tim Kaine, reaffirmed their support for the bill.

The debate over the Iran deal seems likely to turn partisan, poisoned by the start of the 2016 US presidential campaign.

Former Florida governor Jeb Bush, who has not formally announced his candidacy but currently leads the Republican field, lost no time in denouncing the agreement.

A partisan fight could keep Democrats in Obama's corner.

"I'm a co-sponsor of the bill, but if I see this slipping toward pure partisanship and we're going to try to embarrass the President, I'm off the bill," Senator Angus King, an independent from Maine who caucuses with the Democrats, told CNN.