The backing from the Senate’s most powerful Democrat adds even more momentum on Capitol Hill to protect the nuclear agreement. Reid becomes the 27th Senate Democrat to formally endorse the Iran deal, and Obama needs just 34 votes in the Senate to sustain a veto of a resolution of disapproval that is expected in Congress next month.

No Republicans are expected to vote in favor of the nuclear deal. But in recent days, key Democrats have come out in favor of the agreement – including red-state Sens. Joe Donnelly of Indiana and Claire McCaskill of Missouri, and Jewish lawmakers such as Rep. Jerrold Nadler of New York.

In an interview with the Washington Post, Reid said he was “cautiously optimistic” that there would be enough Democratic support to sustain any veto from Obama. His counterpart in the House – Minority Leader Nancy Pelosi of California – has confidently predicted that Obama’s veto won’t be overridden.