WASHINGTON — (CNN) New York Sen. Chuck Schumer, an influential Jewish Democrat who’s poised to assume leadership of his party in the Senate, will oppose President Barack Obama’s nuclear deal with Iran, he announced on Thursday evening.

“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 wrote in a 1,600-word post on the website Medium.

“I will vote to disapprove the agreement, not because I believe war is a viable or desirable option, nor to challenge the path of diplomacy,” he added later. “It is because I believe Iran will not change, and under this agreement it will be able to achieve its dual goals of eliminating sanctions while ultimately retaining its nuclear and non-nuclear power.”

Schumer’s thinking on the deal has been closely watched since the plan to loosen sanctions on Tehran in exchange for access to potential nuclear sites was announced in early July.

He said in public comments over the past several weeks he was going through the deal with a “fine-tooth comb” and received briefings from top administration officials to digest the deal’s inner workings.

His decision quickly provoked criticism from several former White House aides.

Former speechwriter Jon Favreau tweeted: “Chuck Schumer, who said it was a mistake to pass Obamacare, now comes out again the Iran Deal. This is our next Senate leader?”

Schumer’s announcement Thursday came after two Democratic colleagues — fellow New York Sen. Kirsten Gillibrand and Jeanne Shaheen of New Hampshire — both said they would back the agreement, which lawmakers have until mid-September to decide upon.

In the House, at least five Democrats came out in support of the plan on Wednesday following a major speech by Obama defending it.

But a senior House Democrat also came out in opposition Thursday night.

House Foreign Affairs Committee Ranking Member Eliot Engel, Schumer’s New York colleague, also officially announced he was against the deal, despite a one-on-one meeting with Obama last week.

A White House official suggested Thursday that Schumer’s announcement of opposition came only after enough Democratic support was assured to keep the plan intact.

One source close to the New York delegation’s discussions told CNN that Schumer was expected to hold off making his opposition public until the math was clear that Republicans wouldn’t be able to assemble votes for an override, though the source thought Schumer would wait until the fall to announce.

But in recent days a steady stream of public endorsements made supporters more confident they would have a firewall in place to uphold the deal.

A source familiar with Schumer’s decision on Iran told CNN that the senator informed the White House earlier today of his intention to oppose the deal. He had planned to make his decision public Friday. During the first 2016 Republican presidential debate Thursday night, however, the Huffington Post reported that he would be coming out against the deal, and soon after his statement appeared on Medium.

Schumer’s leanings on the deal had been closely watched since the plan to loosen sanctions on Tehran in exchange for access to potential nuclear sites was announced in early July.

Since then, the Obama administration has waged a massive lobbying effort to persuade Democrats to back the plan. The President has vowed to reject any measure scuttling the agreement, and a majority of Democrats are needed in Congress to sustain a potential veto.

Schumer had been eyed as a key vote, both for his prominence as the top Jewish Democrat in Congress and the expectation that he’ll become the Democratic leader in the Senate when Minority Leader Harry Reid retires at the end of his term in 2016.

After his review, he wrote Thursday that worrisome elements of the deal — including the provisions concerning inspections and the formula for sanctions to “snap back” if Iran violates its side of the agreement — led him to his decision.

And he argued sanctions relief for Tehran would only serve to empower a hardline regime.

“To me, 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.

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