The decision by Senator Chuck Schumer to oppose President Obama ’s deal to limit Iran’s nuclear program rattled the Democratic firewall around the accord, but supporters said Democratic defections in New York and South Florida would not be enough to bring down the agreement. Republican leaders in the House and Senate have promised a vote in mid-September on a resolution to disapprove the nuclear accord between Iran and the United States, Germany, Britain, France, Russia and China, which in itself would be a blow to Mr. Obama’s prestige. But to scuttle the Iran nuclear deal, opponents have two high hurdles. They will need 60 votes in the Senate for a resolution of disapproval to overcome a filibuster […]