Representative Jerrold Nadler was being pulled hard from all sides.

Close allies in the pro-Israel community told him a nuclear deal with Iran would doom the Jewish state. His Jewish colleagues in New York’s congressional delegation had all condemned the pact. Then there was the lobbying from President Obama, first with a phone call, followed by a face-to-face talk in the White House — Mr. Nadler’s first such meeting in Mr. Obama’s tenure.

Facing extraordinary pressure, Mr. Nadler on Friday became the lone Jewish Democrat from New York to endorse the president’s proposed agreement with Iran, which would lift economic sanctions in exchange for Iran’s taking steps to abandon a path to nuclear arms.

The decision by Mr. Nadler, 68, a long-tenured liberal whose district is believed to have the largest Jewish population in the country, may make it easier for other House Democrats to support the agreement, including non-Jews who remain undecided on the deal. He has a history of energetic support for Israel, and has played a pivotal role as an emissary between liberal Democrats and more conservative supporters of Israel.

Without Mr. Nadler’s endorsement, proponents of the deal would have faced a shutout of the Jewish members of New York’s congressional delegation, whose views carry considerable weight on issues related to the Middle East and Israel. Senator Chuck Schumer has announced he will vote in disapproval, and so have Representatives Eliot L. Engel, Nita M. Lowey and Steve Israel, who are all senior members of the Democratic caucus.

In an interview, Mr. Nadler said his position on the Iran deal was a wrenching decision, putting him at odds with constituents who fear for Israel’s security. While he called the deal an imperfect option, he said he believed it was the best chance for averting a nuclear-armed Iran.

“What helps Israel and averts another potential Holocaust is whatever is most likely to prevent Iran from getting a nuclear bomb,” Mr. Nadler said. “My conclusion is that this deal — of the available alternatives to us, not what might or should have been — is the best.”

Mr. Nadler’s decision is emblematic of the challenge facing liberal Jews in Congress, and beyond, as they weigh the merits of the agreement. Many feel torn between their hope for avoiding another Middle East war and their support for Mr. Obama, versus their deep relationships with a pro-Israel community intensely opposed to the deal.

But the congressman is in a particularly delicate position: His district stretches down the West Side of Manhattan and into Brooklyn, spanning Jewish communities from the liberal Upper West Side to ultra-Orthodox Borough Park. He said he expected many constituents to be angry about his stance.

“Not only angry — sad, disappointed: ‘We believed in you. We’ve always supported you, and now you’ve abandoned us,’ ” Mr. Nadler said, speaking in a soft and level tone. “That’s very difficult to deal with, not just politically but emotionally.”

Mr. Nadler said he had shared a set of concerns about the deal with the White House and had been pleased by the response. Mr. Obama sent a letter to him this week vowing to strengthen the arrangement with Iran in ways that would not require reopening the deal.

In a statement, Mr. Obama said that he had sought to address Mr. Nadler’s “specific concerns relating to Israeli security and the U.S. commitment to countering Iran’s destabilizing activities in the region.”

“I wanted to respond to the thoughtful questions Jerry raised, and I am pleased that our discussions were ultimately productive,” the president added.

But Mr. Nadler said the lobbying from Mr. Obama, who is viewed by many on Capitol Hill as a relatively aloof figure, had not determined his vote. Mr. Nadler noted that the president’s outreach on the Iran deal was the first time he had received a phone call or personal meeting from him during Mr. Obama’s six and a half years in office.

“I would have voted for this proposal even if he had not made these improvements,” Mr. Nadler said of the president’s letter.

Mr. Engel, the top Democrat on the House Foreign Affairs Committee and a prominent opponent of the deal, said Mr. Nadler would face a backlash over his vote. But Mr. Engel defended his colleague’s credentials as a champion of Israel.

“There will be other issues and other ways that Jerry can express himself, in terms of being a Zionist all these years,” Mr. Engel said, adding: “I won’t second-guess him. He’s got a good heart when it comes to Israel.”

Criticism came swiftly on Friday. State Assemblyman Dov Hikind of Brooklyn, an Orthodox Jewish Democrat who has led demonstrations against the nuclear deal, vowed to support a well-financed Democratic opponent against Mr. Nadler.

“There are millions of dollars that will be there,” Mr. Hikind said. “This is huge, absolutely huge, for people in our community.”

The American Israel Public Affairs Committee, the powerful pro-Israel group that has whipped votes in opposition to the deal, declined to comment on Mr. Nadler’s position.

Mr. Nadler said he was persuaded that the agreement would give international powers the ability to block Iran’s access to weapons-grade uranium and plutonium, perhaps stopping the country from obtaining an atomic bomb permanently. He expressed some reservations about the deal’s effectiveness over the long term, as it lifts certain restrictions on Iran’s activities toward the end of a 15-year agreement.

Still, he said, the worst-case outcome is that the deal “postpones a possibly apocalyptic problem for a dozen years.”

In announcing and explaining his support for the agreement, Mr. Nadler posted a nearly 5,200-word essay online. He lamented the “poisonous rhetoric” surrounding the debate over Iran. Voices on the left, he said, “are making anti-Semitic accusations of dual loyalty or treason” when Jewish lawmakers oppose the deal.

On the other hand, Mr. Nadler said, choosing to support the plan “does not make someone anti-Israel.”

If Mr. Nadler’s declaration sets him apart from some of his influential colleagues, and perhaps from many of his constituents, it is not an entirely unfamiliar position: In 2002, he was New York’s only Jewish member of Congress to vote against authorizing war in Iraq.

Supporters of the nuclear deal with Iran have sought to present it as a similar vote, to forestall the prospect of military action through a negotiated agreement.

In some respects, Mr. Nadler’s district is a snapshot of the deepening divisions among American Jews nationwide, as conservative supporters of Israel have grown more caustic in their attacks on Mr. Obama and his policies, and liberal Democrats become more openly critical of the Jewish state and its leader, Prime Minister Benjamin Netanyahu.

Old-school liberal Zionists, like Mr. Nadler, do not fit easily into either ideological camp. The congressman, who has been entrenched in his seat since 1992, said it was possible that his vote could generate a challenge in the Democratic primary election next year.

Brad Lander, a Democratic city councilman who represents a portion of Mr. Nadler’s congressional district, predicted that Mr. Nadler would weather the political storm. Councilman Lander, who is Jewish, said the congressman has long navigated a “great tradition of dispute within our community.”

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