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.