WASHINGTON — Sen. Charles Schumer abruptly canceled his only public appearance Friday, a day after his announcement that he was opposing the Iran nuclear deal drew blowback from both sides of the aisle.

Ending three weeks of indecision, Schumer disclosed Thursday night, during the Republican presidential debate, that he would vote against the deal when it reaches Congress next month.

Other opponents of the pact called for him to do more to stop it, including lobbying his fellow Democrats to override an expected presidential veto.

“He’s not just another senator. He’s a key leader in the Senate,” said Malcolm Hoenlein, executive vice president of the Conference of Presidents of Major Jewish Organizations.

The timing of Schumer’s announcement caught advocates on both sides by surprise.

He issued a lengthy statement as millions were watching the GOP debate and just hours after his junior colleague, Sen. Kirsten Gillibrand, declared she would vote for the agreement.

In the statement, Schumer said he would discuss his opposition with Democratic fence-sitters but expected them to come “to their own conclusion.”

“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.”

Reporters had hoped Schumer would elaborate during a scheduled appearance Friday at the opening of a brewery in Coney Island, but organizers said he canceled in the morning.

Supporters of the agreement were furious that the Senate’s No. 3 Democrat, who is in line to lead his caucus next year, was on the other side of President Obama’s keystone foreign-policy initiative.

Former White House adviser Dan Pfeiffer tweeted that Schumer’s opposition “will make it hard” for him to lead Senate Democrats.

“He may keep the job, but he will be less effective, less influential and the party will be less successful,” he tweeted.

Liberal advocacy group MoveOn.Org said it was launching “a Democratic Party donor strike” to protest Schumer’s move.