Schumer’s move will come a day after New Hampshire Democratic Sen. Jeanne Shaheen and Schumer’s fellow New York senator, Kirstin Gillibrand, announced their support for the deal. That momentum is blunted by Schumer’s pending announcement. Backers of the deal had hoped that if Schumer decided to oppose the deal, he would hold off until the last minute.

Schumer’s support of a war footing over diplomacy puts him at odds with the Democratic caucus he intends to lead next term, though it is consistent with the position he has long taken.  

As the soon-to-be leader of his party in the Senate, Schumer’s decision to directly fight President Barack Obama on the biggest foreign policy achievement of his presidency is a bold, but not surprising, move. Schumer has long been more hawkish on foreign policy than some of his fellow Democrats. In 2002, he voted for the Authorization for the Use of Military Force in Iraq. In 2006 he backed John Bolton’s nomination to serve as George W. Bush’s ambassador to the United Nations, reportedly telling a Senate Democratic caucus meeting that “a vote against Bolton was a vote against Israel.” Bolton is an ardent foe of negotiation with Iran.