WASHINGTON — New York Sen. Chuck Schumer, the No. 3 Senate Democrat and next-in-line party leader, said late Thursday that he is breaking with President Barack Obama and will oppose the Iran nuclear deal.

“After deep study, careful thought and considerable soul-searching, I have decided I must oppose the agreement and will vote yes on a motion of disapproval,” Schumer said in a statement weeks before he will cast a vote.

Schumer, a leading Jewish Democrat, is the first senator of Obama’s party to step forward to oppose the deal. His announcement came just hours after two other Senate Democrats — New York’s Kirsten Gillibrand and New Hampshire’s Jeanne Shaheen — announced their support for the international accord.

The deal, struck last month with Tehran and Western powers, would curb Iran’s nuclear program in exchange for billions of dollars in relief from crippling sanctions.

Schumer’s decision is a blow to the administration, though it remains to be seen whether other Democratic lawmakers will follow the New York senator. His split with Obama is remarkable for a senior leader in line to replace Senate Minority Leader Harry Reid, D-Nev., after he retires at the end of next year.

Schumer’s decision also puts him at odds with the Democrats’ likely presidential nominee, Hillary Rodham Clinton, who has cautiously embraced the deal. The Senate’s No. 2 Democrat, Dick Durbin of Illinois, supports the accord and has been working hard to persuade lawmakers to do the same.

READ: Is Obama’s Iran deal rhetoric working?

The administration, which has lobbied intensely for the pact, had secured the backing of more than a dozen Senate Democrats and more than two dozen House Democrats, including Minority Leader Nancy Pelosi, D-Calif. Republicans, who control the House and Senate, are uniformly opposed to the deal.

Schumer signaled that he wouldn’t lobby hard against the accord. The House and Senate will begin debate on a resolution of disapproval when lawmakers return to Washington on Sept. 8 after their August recess. The administration needs Democratic support to sustain a widely expected veto by Obama of any resolution of disapproval.

“There are some who believe that I can force my colleagues to vote my way,” he said. “While I will certainly share my view and try to persuade them that the vote to disapprove is the right one, in my experience with matters of conscience and great consequence like this, each member ultimately comes to their own conclusion.”

Schumer has been under pressure as a congressional ally of Israel, leading fundraiser and strategist for his party, and lawmaker from a state that is home to more than a million-and-a-half Jews.

“The very real risk that Iran will not moderate and will, instead, use the agreement to pursue its nefarious goals is too great,” he said in opposing the pact.

Schumer said he based his decision on the nuclear and non-nuclear elements of the accord and on the question of “Are we better off with the agreement or without it?”

He complained that the pact does not allow inspections “anywhere, anytime” and that the United States cannot demand inspections unilaterally.

Schumer joins a handful of Jewish Democrats who have announced their opposition — Reps. Steve Israel and Nita Lowey of New York and Ted Deutch of Florida. Michigan Rep. Sander Levin, however, has endorsed the deal.

The powerful pro-Israel lobby American Israel Public Affairs Committee is vehemently opposed to the deal, which Israeli Prime Minister Benjamin Netanyahu has denounced as undermining the security of Israel and the region. Opponents of the pact have targeted Schumer in campaign-style ads.

The media-friendly Schumer made the announcement through the blog Medium, not in a high-profile speech on the Senate floor like several of his colleagues. His statement also was posted as much of the political world was focused on the first Republican presidential candidate debate.

The announcement came hours after the Senate left Washington for its recess. The move spares Schumer weeks of intense lobbying from proponents and foes of the deal.