"I told the president the day before and I don't think he leaked it, but maybe somebody in the White House did, maybe somebody else did. I'm not pointing fingers," Schumer told reporters after a speech at New York University on Tuesday.
The White House has denied being behind the media leak.
Schumer's comments are the latest salvo between the New York senator and a White House at odds on a centerpiece of Obama's foreign policy legacy. Former Obama aides and progressive outside groups have harshly attacked Schumer since his position became public and have questioned whether he's fit to be the next Senate Democratic leader.
Sen. Dick Durbin of Illinois, the No. 2 Senate Democrat, threw cold water on this idea Tuesday, telling The Hill that he still supported Schumer. So far, no senators have come out against him as the next Senate Democratic leader.
Schumer was long thought to oppose the deal, but the timing of his statement of opposition -- ahead of Congress's summer recess, when members are expected to come under intense lobbying from both sides, rather than closer to the vote when revealing his position would be less influential -- has particularly distressed the White House.
Congress will hold a vote on the Iran deal in September. While the White House is confident that they have enough votes to sustain an expected presidential veto should Congress reject the deal, they are counting every body.
Schumer raised the issue of a possible White House leak himself Tuesday in response to a question about the timing of his lengthy post on Medium that outlined his concerns about the deal. The leak of his decision came out in the middle of the Republican presidential debate last Thursday night, presumably when it would receive less attention, and the Medium piece appeared soon after.
The New York Democrat said he planned to publish the piece the day after the GOP debate, but his hand was forced by the leak.
Even though he was the one to aim the blame at the White House, Schumer appeared to downplay the episode Tuesday, saying, "Things always leak in Washington. So I don't begrudge anybody for doing it."
And he also minimized the split with the President. "The President and I had a very serious discussion about it," he said. "We like each other, respect each other, work together on 95% of the issues."
The President's allies, though, are taking the breach seriously. David Plouffe, a former senior official in the Obama White House, continued the Democrat-on-Democrat war against Schumer late on Monday when he tweeted out a link to a story quoting Schumer urging the administration to try to negotiate a better deal.
"Mitch McConnell will have a field day with this kind of naïveté. We will miss Harry Reid," Plouffe tweeted.
The current Senate Democratic Leader, Harry Reid of Nevada, announced his retirement in March and Schumer quickly locked up commitments from fellow Democrats to ascend to his post in January 2017.
White House Press Secretary Josh Earnest, however, denied the White House had any part in the leak at a press briefing Friday.
An administration source also told CNN on Tuesday that they learned about Schumer's opposition from sources on Capitol Hill before the senator informed them, implying several people knew of his position and raising the specter that others had leaked the information.
Schumer said fellow Democrats understand where he stands and aren't telling him they won't back him because of it.
Schumer said Tuesday that he plans to try to "persuade" undecided Democrats to help him block the deal but insisted he wasn't twisting any arms on what he called "a vote of conscience."
"Certainly I'm going to try and persuade my colleagues that my viewpoint is right. But anyone who thinks you can force somebody to vote with you in the Senate doesn't understand the Senate."
Missouri Democratic Rep. Claire McCaskill told Yahoo News' Katie Couric that Schumer is "one of my best friends" and said she spoke with him since he announced his opposition to the deal, but that he didn't lean on her vote against it.
McCaskill added that Schumer "checked in" with her and that "he understands that this is a tough call."
Schumer said any effort to engage in "some kind of horse-trading" in return for a vote on the agreement would be "counterproductive."
Meanwhile, in Israel opponents of the deal were also trying their hand at persuasion.
On the heels of a visit by Democrat members of Congress, Israel's President Reuven Rivlin hosted a delegation of 36 House Republicans on Tuesday. The group was led by House Majority Leader Kevin McCarthy of California, a vocal critic of the Iran deal.
"We fear that the agreement with Iran is a first step in the legitimization of Iran's policies and strategies, and only acts to further destabilize a chaotic region," Rivlin told the lawmakers.