Opposing Iran Nuclear Deal, Chuck Schumer Rattles Democratic Firewall

The decision by Senator Chuck Schumer of New York to oppose the Iran nuclear deal has rattled the Democratic bulwark around the accord, emboldened the deal’s opponents in both parties, and set off a wave of condemnation from liberals for the man who hopes to lead Senate Democrats in the next Congress.

But supporters of the accord said on Friday that Democratic defections would not be enough to bring it down.

To scuttle the deal, opponents have two high hurdles. They will need 60 votes in the Senate for a resolution of disapproval to overcome a filibuster by supporters of the accord. If the opponents get that, the president will veto the resolution. The opponents would then have to secure the votes of two-thirds of the lawmakers in both chambers to override the veto.

“It is less likely than not that Congress is going to override,” said Representative Brad Sherman of California, a senior member of the House Foreign Affairs Committee, who came out against the nuclear accord on Friday. “That happens almost never, and even less often on foreign policy.”

Mr. Schumer’s voice is powerful, and his politics are wily, but he alone cannot stop the international agreement, which Republican leaders in the House and the Senate have promised to bring to a vote in mid-September.

“It is a decision that many people, including our whip operation in the House, had been watching closely, no question about that,” said Representative David E. Price, Democrat of North Carolina and a member of the House’s informal team whipping up support for the accord. “I wouldn’t underestimate the importance of Chuck Schumer, but he’s not likely to reverse the situation.”

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