Democrats clinched the crucial Senate votes yesterday to block passage of a disapproval resolution against the Iran nuclear accord, an outcome that would be a major win for President Barack Obama against united Republican opposition.

But supporters of the deal stopped short of declaring victory amid the fast-paced developments as lawmakers returned to Washington from their five-week summer recess.

That was because it remained uncertain whether all 41 Democratic and independent senators now on record in favour of the deal would also support a filibuster to block a final vote on the disapproval resolution.

Still, the complicated machinery of Congress was turning in favour of the president on his top foreign policy priority, despite GOP control of both the House and the Senate.

Already supporters of the deal have the votes in hand to uphold Obama’s veto of a disapproval resolution, should that become necessary.

Blocking the disapproval resolution with a filibuster, while ideal from the White House view because it would spare Obama from a veto fight, would not change the ultimate outcome.

“This agreement will stand,” Senate Minority Leader Harry Reid, D-Nev, said on Tuesday morning at the Carnegie Endowment for International Peace. “America will uphold its commitment and we will seize this opportunity to stop Iran from getting a nuclear weapon.”

White House Press Secretary Josh Earnest made clear the administration’s preference for the Senate to block the disapproval resolution before it could reach Obama’s desk.

As the day began yesterday Senator Joe Manchin of West Virginia announced his opposition to the deal, a surprise “no” vote from a moderate Democrat who had sounded like he favoured the pact aimed at curbing Iran’s nuclear ambitions.