Sen. Chuck Schumer will oppose the Iran deal, a decision that deals quite a blow to President Barack Obama and the Democrats working to rally support for the deal.

“Advocates on both sides have strong cases for their point of view that cannot simply be dismissed,” Schumer wrote in a lengthy post Thursday on the website Medium. ”This has made evaluating the agreement a difficult and deliberate endeavor, and after deep study, careful thought and considerable soul-searching, I have decided I must oppose the agreement and will vote yes on a motion of disapproval.”

Obama needs 34 votes to sustain a veto if Congress does not approve the deal.

The senior senator has been on the fence for weeks, and his decision will make it that much harder for Democrats in Congress, led by Rep. Nancy Pelosi in the House, to rally support for the deal.

“Anybody whipping has never put Schumer in the ‘yes’ column,” Democratic Rep. Jan Schakowsky, who’s working with Pelosi to rally support, told The New York Times. “But the calculation still is we’ll have the votes [without him].”

Support for the deal had gained momentum Thursday, as Democratic Sen. Kirsten Gillibrand announced her support. Twelve Democrats and one Independent senator have announced their support for the deal.

“There are some who believe that I can force my colleagues to vote my way,” Schumer added. “While I will certainly share my view and try to persuade them that the vote to disapprove is the right one, in my experience with matters of conscience and great consequence like this, each member ultimately comes to their own conclusion.”

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