WASHINGTON (AP) — Democrats struggled Tuesday for the votes to block a disapproval resolution against the Iran nuclear deal as one undecided senator announced a surprise "no" vote shortly after lawmakers returned from a five-week summer recess.

Democratic Sen. Joe Manchin of West Virginia had spoken supportively of the international deal to curb Iran's nuclear program in exchange for billions of dollars in relief from economic sanctions.

Manchin said he fears the pact would not keep Iran from becoming a nuclear weapons state over the longer term. "I cannot gamble our security, and that of our allies, on the hope that Iran will conduct themselves differently than it has for the last 36 years," Manchin said in a statement.

The announcement left Democratic supporters three votes shy of the 41 votes they would need to bottle up the disapproval resolution in the Senate with a filibuster later this week. Four Democratic senators have yet to announce their stance and more announcements were coming later Tuesday.

Still, the ultimate outcome was not in doubt. Even if the disapproval resolution passes, President Barack Obama has promised to veto it, and Democrats have the votes in hand to uphold his veto.

"This agreement will stand," Senate Minority Leader Harry Reid, D-Nev., said in a speech 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."

But the White House and Democratic allies led by Reid would like to spare Obama the embarrassment of having to use a veto to rescue his top foreign policy priority. They were waiting to hear from the undeclared senators: Ron Wyden of Oregon, Maria Cantwell of Washington, Richard Blumenthal of Connecticut and Gary Peters of Michigan.

The agreement struck by Iran, the U.S. and five world powers in July will 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.

Republicans who control the House and Senate strongly oppose the pact, saying it makes dangerous concessions to Iran, and hope to push through a resolution of disapproval this week.

Leaders of Israel have been strongly lobbying against the deal they say could empower Iran, but had succeeded in winning over only three Senate Democrats, albeit all of them prominent figures — Chuck Schumer of New York, Robert Menendez of New Jersey and Ben Cardin of Maryland, the top Democrat on the Senate Foreign Relations Committee. Manchin added his name to that list Tuesday.

But the majority of Democrats have swung behind the president, and predictions that the issue would dominate discussion during Congress' August recess never came to pass as political headlines were largely overtaken by Donald Trump's presidential candidacy. The two topics will converge on Wednesday, though, when Trump joins Texas Sen. Ted Cruz for a rally to oppose the deal — the same day Hillary Rodham Clinton delivers a speech supporting it.

The deal sets Iran back so that it is at least a year away from being able to produce enough nuclear material for a weapon, before the restrictions ease after a decade. Iran is currently assessed to be only 2 to 3 months away from being able to enrich enough uranium for a bomb, if it decides to do so.