WASHINGTON — Pressing their advantage, the White House and insistent Senate Democrats locked up the votes Tuesday to frustrate attempts by outraged Republicans to pass a legislative rebuke to the Iran nuclear accord.

Four previously undeclared Senate Democrats – Richard Blumenthal of Connecticut, Ron Wyden of Oregon, Gary Peters of Michigan and Maria Cantwell of Washington state – announced their support for the international agreement in a coordinated burst. That pushed supporters to a 42-vote total, one more than necessary to block a GOP disapproval resolution with a filibuster, and prevent a final vote.

“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.

Blumenthal, Wyden and Peters 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 presidential candidate Donald Trump 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 42 senators now on record in favor of the Iran deal would hold together on the procedural maneuvers necessary to mount a filibuster. 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 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.

“If we have to go through the procedural charade of a veto, and a vote to sustain the veto, it will be embarrassing for this administration and this country,” said Sen. Chris Murphy, D-Conn., who’s been involved in coordinating support for the agreement. “So 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.”

At the White House, press secretary Josh Earnest indicated the administration shares that preference as the administration officially issued a veto threat.

Rejection of the deal “would be the worst of all possible worlds, leaving us in a position of weakness, not strength,” the administration said in a statement.

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.

“The Senate should not hide behind procedural obfuscation to shield the president or our individual views,” said Senate Majority Leader Mitch McConnell, R-Ky.

McConnell jousted on the Senate floor with Senate Minority Leader Harry Reid, D-Nev., over the terms of the debate as each side sought to frame the other as obstructionist.

“This agreement will stand,” Reid said.

Sen. Bob Corker of Tennessee, top Republican on the Foreign Relations Committee, delivered an impassioned speech that was a broadside against the Obama administration.

“We know we have no policy in the Middle East,” Corker said. The Iran deal “ends up being our de facto policy.”

On the other side, the No. 2 Senate Democrat, Dick Durbin of Illinois, said, “What the president is trying to do is something that I believe should be the starting point in every critical foreign policy decision: use diplomacy.”

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.

Even as they faced the prospect of legislative defeat, Republicans tried some last-ditch maneuvers to avoid that outcome.

In the House, Republican Rep. Peter Roskam of Illinois offered a resolution to delay the vote, arguing that the clock on the congressional review period shouldn’t start until lawmakers get more information on side agreements negotiated with Tehran. Speaker John Boehner, R-Ohio, also pointed to a letter from 15 GOP governors saying they plan to maintain state-level sanctions against Iran.

Nearly lost in the developments, the anti-deal side picked up one surprise adherent: Sen. Joe Manchin of West Virginia, a moderate Democrat who had widely been counted in the “yes” column after making positive comments about the deal, became the fourth Senate Democrat to oppose it. “I’ve always said if I can’t explain it back home, I can’t support it,” Manchin said.

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. Sounding a familiar theme, they said the agreement is about verification, not trust.

“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.”