WASHINGTON -- The White House and insistent U.S. Senate Democrats locked up the votes Tuesday to frustrate attempts by outraged Republicans to pass a legislative rebuke to the Iran nuclear accord.

Three previously undeclared Senate Democrats -- Richard Blumenthal , Ron Wyden and Gary Peters -- announced their support for the international agreement in a co-ordinated burst. That pushed supporters to the crucial 41-vote total that would allow them to block a Republican disapproval resolution with a delaying tactic and prevent a final vote.

The agreement struck by Iran, the U.S., China, Russia, France, Britain and Germany in July would provide Iran hundreds of billions of dollars in relief from international sanctions in exchange for a decade of constraints on the country's nuclear program. The deal aims to keep Iran at least a year away from being able to produce enough nuclear material for a weapon.

"There is no better deal available now," declared Blumenthal, one of the Senate's Jewish Democrats, announcing his support for an accord that is strongly opposed by Israeli leaders as well as Republican senators.

The three lawmakers were among just a handful of undeclared senators and were all considered possible "no" votes. Coming on the first day of Congress' fall session after a five-week summer recess, their announcements were a dramatic start to what promises to be a bitter, partisan debate on the deal aimed at curbing Iran's nuclear program.

The debate will take on some of the trappings of a political circus Wednesday with Republican presidential candidates Donald Trump and Sen. Ted Cruz preparing to headline an anti-accord rally outside the Capitol.

It will play out at the start of a hectic three weeks on Capitol Hill. Congress must also find a way to keep the government funded past the end of the fiscal year Sept. 30, and lawmakers will host a historic visit from Pope Francis.

It remained uncertain Tuesday if all 41 senators now on record in favour of the Iran deal would hold together on the procedural manoeuvrs necessary to delay debate. And the developments didn't change the ultimate outcome, which has been clear for days: Even if the disapproval resolution should pass the House and Senate this week, President Barack Obama would veto it, and Democrats have the votes in hand to sustain his veto.

But Obama and his Democratic allies now have within reach the possibility of stopping the resolution without a messy veto fight, despite the unanimous opposition of the Republicans who control both the House and the Senate.

"I think it's cleaner, simpler and much better for American credibility around the world if the motion to disapprove doesn't get past the Senate, " said Democratic Sen. Chris Murphy, who's been involved in co-ordinating support for the agreement.

At the White House, Press Secretary Josh Earnest indicated the administration shares that preference.

"We certainly would expect that those members of Congress who support the agreement to take the necessary steps in Congress to prevent Congress from undermining the agreement," he said.

Republican opponents on Capitol Hill complained angrily about Democratic attempts to avoid a final vote on the disapproval resolution. The pro-Israel lobby AIPAC was also pressuring senators behind the scenes to allow a final vote even if they intend to oppose the disapproval resolution, creating uncertainty as to whether parliamentary delaying tactics would succeed.

Joined by military veterans, several dozen House Democrats gathered on the steps of the Capitol Tuesday evening to show their support for the deal with Iran.

"Tonight we stand, members of Congress, on the steps of the Capitol, as members totally committed to preventing a nuclear-armed Iran," said House Democratic leader Nancy Pelosi. And, she added, "as friends of Israel, dedicated to strengthening Israel's security."