People who have spoken with the senior New York senator believe the pressure campaign is having an effect: They say there is a growing sense inside and outside the Capitol that Schumer will vote against the deal when the Senate considers it in September. The bigger question many have now is this: How hard will he push against it?

Schumer is one of about 15 Democratic senators who will decide the fate of President Barack Obama’s Iran nuclear deal in Congress. The president can afford to lose no more than a dozen Democrats on the Senate floor, and as the next Democratic leader, Schumer may be the most critical of them all.

In an interview with POLITICO, Schumer insisted he’s still weighing his vote. He said he would decide based on the merits of the deal, not lobbying from either side.

“I haven’t made up my mind,” said Schumer, who is in line to be the first Jewish Senate leader next Congress. “There are expectations all over the lot. I’m doing what I’m always doing when I have a very difficult decision: Learning it carefully and giving it my best shot, doing what I think is right. I’m not going to let pressure or politics or party get in the way of that.”