Since Iran and world powers struck a historic nuclear deal in July, all eyes have fallen on Senator Chuck Schumer, the Democrat from New York. Last Thursday, the notoriously camera-hungry Schumer announced his opposition in a lengthy online statement. That he would become the first and only Senator, so far, to break with his party and his president didn’t come as a surprise: Schumer has long approached the Middle East with what can properly be called an unreconstructed pro-Israel hawkishness.

That hawkishness permeated Schumer’s statement; it proclaimed a thoughtful deliberation, gave plaudits to the Obama administration for its hard work, then proceeded to throw up the standard right-wing pro-Israel talking points against the nuclear deal. “Schumer’s missive came across a bit like your crazy uncle who gets his opinions from talk radio and wants to set you straight at Thanksgiving,” wrote the nonproliferation expert Jeffery Lewis in Foreign Policy, before dismantling Schumer’s technical objections. (Other nuclear experts, too, have questioned Schumer’s points against the deal.)

The timing of Schumer’s announcement reportedly riled the White House: Coming as it did just before Congress’s August recess, Schumer’s rejection of the deal shifts the attention away from himself and onto other undecided members, despite several prominent Democrats recently coming out in favor of the accord. That’s why, though Schumer’s objection to the deal was not unexpected, it nonetheless constituted a major development; in part for the prospects of the nuclear deal itself and in part for what his dead-end Middle East hawkishness means for the future of the Democratic Party—which Schumer hopes to lead in the upper chamber when Senate Minority Leader Harry Reid (NV) retires this winter.

Accordingly, Republican hawks are heartened on both counts. “I hope we see the re-emergence of Joe Lieberman Democrats,” said GOP presidential hopeful Senator Ted Cruz (TX), referring to the neoconservative former Connecticut senator whose Iraq war vote cost him a Democratic primary—and a spot in the party—in 2006. “They have been an endangered species in the United States Congress, and it is my hope that with Senator Schumer coming out that he will take a significant role leading and encouraging his fellow Democrats to stand together.”

Many observers assumed Schumer’s absence from the Sunday shows last weekend indicated he won’t be working to gain support for his position against a deal. But a Democratic Senate staffer, who asked to be unnamed to preserve their boss’s relationship with Schumer, told me unequivocally that the New York Senator has been making calls. “Schumer is whipping,” the staffer said, confirming Schumer’s remarks to the press today.

While Schumer is engaged in elite activism, however, a bevy of grassroots progressive groups are pushing back hard. More than 76,000 people signed a petition on the website Daily Kos denouncing Schumer’s decision and CREDO, another progressive group, amassed more than 160,000 signatures on a petition criticizing Schumer’s decision. MoveOn Action, the anti-war group, released its own statement in response to Schumer and launched a campaign to withhold donations from Democrats who side with the would-be Democratic Senate leader. MoveOn Action executive director Ilya Sheyman said that, as of Sunday evening, the group had amassed pledges to withhold more than $8 million in political giving, and expected to hit its goal of $10 million withheld this week.

Though the MoveOn statement makes reference to Schumer’s leadership bid, Sheyman made clear the group’s “north star” was getting the Iran agreement through Congress: “Our focal point isn’t a political battle, it’s how do you keep this deal in place,” he said, adding that the donation boycott would “send a clear message to other elected officials who haven’t come out in favor of a deal.” Nonetheless, the stakes for the Party’s future may hang in the balance: “If you want to be a leader in the Democratic Party, you have to stand up for diplomacy and not a return Bush-era foreign policy.”

The Democratic Senate staffer suggested, however, that planting a hawkish pro-Israel flag may be exactly what Schumer has in mind. “He’s basically come out very early and said to the entire Democratic caucus, ‘Israel issues are just off the table,’” the staffer said. “He’s saying, ‘Any time anything like this comes up, you can count on me to be in the extreme pro-Israel position.’ That’s a tell for the future.”

Laying down that marker, though, doesn’t necessarily mean killing Obama’s Iran deal. The staffer told me Schumer knows that, according to the procedure laid out in the Iran Nuclear Agreement Review Act, if either chamber passes a resolution disapproving of the deal and Obama, as he has promised to, vetoes it, the bill would then go back to Congress, where only one chamber would need to sustain the veto for the deal to stand. If that vote goes to the House—where Democratic leader Nancy Pelosi (CA) has argued forcefully in support of the deal—first, then the Senate may never see the agreement again.

A Schumer bundler who supports diplomacy told me he suspected the New York Senator was counting on it. “I would hope that someone who aspires to be majority leader in the next round would not knowingly undermine his own party’s president if he thought the deal was at risk,” said the donor, who asked not to be named to preserve his access to Schumer. Despite that likely calculation, the donor added, “I’m disappointed with his decision to stand against a deal.”

With Pelosi marshaling support for the Iran accord in the House, then, it seems unlikely to me that the deal will be upended by Schumer’s opposition. But his position might yet become kindling that fuels discontent among Democratic Senators on his future leadership role. “The other thing that might be his undoing,” the Senate staffer told me of Schumer’s leadership prospects, “is the government funding fight that we’re going to have after the Iran debate. Schumer is positioned, because he’s a deal-cutter, right in the center of it as a guy who wants to triangulate out of this.” Bucking the White House and his fellow Democrats for the second time in a row, only to side with Republicans, on a major issue would be a “dangerous path,” the staffer said. “Then, if that comes on the heels of Iran, you’ve got a situation where it’s like, What the hell is Schumer up to?”

Still, Schumer’s decision has riled up the Democrats’ anti-war base against him, and they’re poised to be a thorn in his side. “What we saw last week was that Chuck Schumer, a senior Democratic leader—someone who had gotten it wrong and voted for the war in Iraq—once again got it wrong and came out against this agreement,” said Sheyman. “This is a defining moment that will live with elected officials over their career.” Sheyman harkened back again to Joe Lieberman: “If you’ll remember, Joe Lieberman was defeated in a Democratic primary over the [Iraq] war.” He added, “When Ted Cruz thinks you did the right thing, it might be a moment to reconsider the company you keep.”