His decision is considered key because of his standing in the pro-Israel and Jewish communities and because of the doubts he'd expressed about the agreement. It indicates that there could be few remaining undecided Democrats who are likely to come out against the deal.
Nadler's announcement came after Obama sent him a lengthy letter reassuring the congressman about his continued commitment to stopping Iran from obtaining a nuclear weapon by any means necessary, "including military means."
In the wake of loud Republican criticism of the deal, intensified by a controversial draft agreement allowing Iran a role in the inspection of the sensitive Parchin military site, and the recent defection of two high-profile Democratic senators
, the White House has upped its outreach to wavering members.
In addition to the letter to Nadler, Obama on Wednesday penned an op-ed
supporting the deal that appeared in approximately 30 newspapers nationally and has scheduled a webcast with members of the American Jewish community for next Friday.
Congress will vote on whether to reject the deal in September, a measure the White House has promised to veto. Republicans need 44 House Democrats and 13 Senate Democrats to join them to override a veto, and Nadler's support combined with that of three more Senate Democrats in the past three days has dampened their chances of reaching that goal.
Republican 2016 presidential candidates have also warned that should the deal go through, they could stop implementation, as the agreement won't be a binding treaty. Most Democratic candidates have lined up behind the deal, but former Virginia Sen. Jim Webb told CNN Thursday he was opposed, in part because it didn't require congressional ratification -- the measure being considered is one of approval or disapproval that is much easier for the administration to prevail on.
In announcing his support Friday, Nadler said that after careful study he had decided that the deal, "for all its flaws, gives us the best chance of stopping Iran from developing a nuclear weapon."
He noted that, "I bring to my analysis the full weight of my responsibilities as a member of Congress, and my perspective as an American Jew who is both a Democrat and a strong supporter of Israel."
He also took both sides to task: "I have sought to ignore the political pressures, as well as the demagoguery and hateful rhetoric on both sides that I think has been harmful to the overall political discourse."
After Nadler's announcement, Obama issued a statement thanking the congressman for his support.
Nadler had publicly agonized over the process of determining which way to go.
"What I do know is that no member of Congress should take this decision lightly," he wrote this month in an op-ed for the Jewish Telegraphic Agency.
Nadler's priority in his vote is the security of the United States and of Israel, its loyal ally, he said.
'Including military means'
To counter nail-biting on Capitol Hill over the Iran nuclear deal, particularly nerves frayed over Israel's security, Obama used his open letter to Nadler to address many of the remaining questions on the minds of wavering legislators.
Obama was emphatic about how far he'd go to keep Iran from a nuclear weapon.
"As I have repeatedly emphasized, my Administration will take whatever means are necessary to achieve that goal, including military means," he wrote to Nadler.
This goes for "the life of the deal and beyond," the letter read. The President dedicated paragraphs to avowing the financial and strategic muscle his administration has put behind Israel's defense.
In the tug-of-war for legislative sign-off on the Joint Comprehensive Plan of Action -- the name for the anti-nuke deal --- Nadler could be seen as the red mark in the middle of the rope.
His congressional district covers much of Manhattan and a chunk of Brooklyn and has one of the country's largest Jewish constituencies. It also includes the area of the World Trade Center, the main site of the Sept. 11, 2001, terror attacks.
Yet despite tremendous pressure in the aftermath of 9/11 to go along with the Iraq war effort, Nadler voted against "the Bush administration's crusade against Saddam Hussein," he wrote in his op-ed. "I was the only Jewish member in the New York delegation to go against the president."
Obama vs. Netanyahu
The Obama administration is tugging hard at one end; on the other are Republicans and Israeli Prime Minister Benjamin Netanyahu, who has employed harsh rhetoric on the international stage and dispatched his government's emissaries to lobby against the deal.
He visited Capitol Hill to give a speech against the deal in March, and this summer groups of U.S. legislators have gone to Israel to visit Netanyahu and hear his concerns directly.
Though many objected to Netanyahu's public address to Congress blasting the deal -- and the White House saw it as a direct affront -- Nadler defended the Israeli government's participation in the Iran debate.
"I disagree with those who suggest that Israel's government or people must not interfere in seeking to shape American decisions on these issues, and I see such statements as a means of silencing an important part of the discussion," he wrote in his announcement Friday. "Israel and Israelis have an absolutely legitimate right to be concerned, given the existential threat they face, and to articulate that concern openly within the American political debate."
He also criticized the tenor of a debate which has seen those on the extremes accuse some Jewish supporters of the deal of betraying their faith and Israel and, on the other side, charges of treason and putting the interests of Israel before America by Jewish members of Congress who oppose the agreement.