Senator Chuck Schumer, considered the lynchpin in President Obama's strategy to defeat opposition to his Iran deal, is apparently leaning against supporting the agreement, according to Politico.

Schumer is being inundated with emails from an organized effort to convince him to oppose the deal, and anti-agreement forces have already spent hundreds of thousands of dollars on ads in the New York area trying to convince constituents to contact the senator and urge him to oppose the president.

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.”

He wouldn’t say if he would forcefully advocate his position once he makes his stance clear.

“I’ve got to first decide how I’m voting,” Schumer said.

Opponents have been much louder than supporters. If that trend continues over the break at town hall meetings, it will only amplify pressure on swing Democrats to vote against the deal.

Sen. Chris Coons, who was personally lobbied by President Barack Obama and national security adviser Susan Rice to back the deal during a trip to Africa in July, said the view of the accord was about evenly split in his home state of Delaware in the first few days after the announcement. But the Democrat now says telephone calls against the deal outnumber those in favor by 10-to-1 in his state, an avalanche of opposition he has no choice but to listen to.

“I am a Democrat, and I would like to be able to support this agreement,” Coons said. “But I have serious reservations about it.”

Schumer's vote against the deal would give cover to the other dozen or so Democrats inclined to vote against the agreement.  And depending on how hard he lobbied against it, he could personally swing five or six Democrats his way.  Conversely, if he supports the deal, the resolution opposing the agreement will be doomed after a presidential veto.

Internal Democractic politics is playing a bigger role in this debate than many thought.  With Schumer in line to become the next party leader in the Senate in 2016, some back-bench Democrats might want to curry favor with the New York senator by siding with him on the deal.  The more Democratic members oppose the deal, the easier it becomes for fence-straddlers to vote against it.

There are about 40 days to go before Congress must vote on the agreement.  The White House will steadily amp up the pressure on Schumer, convincing key donors and advisors to urge him to vote "no."  Opponents of the deal will continue to campaign among the grassroots and intensify the TV ad buys. 

And if you're familiar with Schumer, you know he's loving every minute of all the attention he's getting.