Senator Chuck Schumer (D-N.Y.) said Thursday night that he can’t support the Iran nuclear deal. Will he bring other Dems with him? REUTERS/Joshua Roberts/Files

Sen. Charles E. Schumer’s announcement that he will oppose the Iran deal was surprising only in its timing, unfolding as it did during the Republican presidential debate on Thursday night.

Many thought the likely “no” voter might stay silent for awhile in deference to the White House’s aggressive push for congressional support.

But now that the New York Democrat’s opposition is out in the open, the question is this: will it ultimately hurt Obama or Schumer more, the former in his quest to sell the deal and the latter in his bid to become the highest-ranking Senate Democrat?

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Schumer wasn’t the only lawmaker to come out against the deal Thursday night – so did Rep. Eliot Engel (D-N.Y.), another skeptic and the House’s ranking member on its Foreign Affairs Committee.

But Schumer, as the highest-ranking Jewish Democrat in Congress and the presumed leader of Senate Democrats once Harry Reid (D-Nev.) retires at the end of next year, carries a much larger megaphone. His opposition could spell trouble for a deal that is already challenging to sell to a skeptical Congress, even among members of Obama’s own party.

If the Iran deal falls apart, it would spark a period of uncertainty, as the world observes how an Iran freed of international sanctions but unfettered by the confines of a broken nuclear deal behaves on the world stage. Another unknown is how, in such circumstances, the United States might respond to perceptions that Iran is nearing nuclear weapons’ readiness.

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Schumer’s colleagues – including undecided ones – readily admitted over the last several weeks that the senator’s opinion on the deal was influential.

But in the end, this comes down to a hard game of numbers.

Obama needs to keep either the House or the Senate from forming a two-thirds majority against the deal to keep the agreement alive, as the Republican-led Congress is expected to deliver the president a “no” vote he will then veto.

Over the last week, the trend in the Senate has been Democrats declaring their support for the deal. Prior to Schumer’s announcement Thursday, a pair of Senate Democrats  — fellow New Yorker Kirsten Gillibrand and Jeanne Shaheen of New Hampshire — also said they would support the Iran agreement.

Obama needs 34 senators on his side, but right now, only 13 are absolute “yes” votes – though another 14 are expected to ultimately vote in favor as well. But that’s still seven shy of what the president needs to override a veto. And a good 15 senators – all Democrats, plus Sen. Jeff Flake (R-Ariz.) – are still either undecided or playing their cards extremely close to the chest.

In the House, the math is less clear. Obama would need 146 votes to maintain his veto there – and at first glance, his chances seem good: Only 8 of the 188 House Democrats have come out against the deal, and he has Minority Leader Nancy Pelosi (D-Calif.) working hard to come up with the needed support. Already, 34 House Democrats have come out for it.

But not enough members have declared themselves to get a full read on the House state of play – and a bunch of newer Democrats may be getting the tough sell on a trip to Israel right now sponsored by AIPAC’s charitable arm.  Only one lawmaker known to be on the trip – Rep. Seth Moulton (D-Mass.) – had previously declared his support for the deal.

Over the past few weeks, senators especially were shuttled through a series of high-level briefings, both in public and behind closed doors, to examine the deal, from security concerns to inspections regimes, and collect information on everything from the nuclear fuel cycle to the negotiating process.

And the true undecideds have yet to break one way or another. Many claim that the longer the briefing process, the more questions arise.

The majority of undecided senators, like Chris Coons (D-Del.), Claire McCaskill (D-Mo.) and Joe Donnelly (D-Ind.), are rank-and-file members who seem determined to either keep their own counsel or stick to their own review process before making a personal decision on the matter. But some leading figures – such as Sen. Ben Cardin (D-Md.), the ranking member on Senate Foreign Relations – are also on the fence. Cardin could arguably sway an even greater flock than Schumer once he makes a decision.

Cardin, for his part, has not said when he will declare himself. But he has been pushing for the administration to provide the text of side agreements between the International Atomic Energy Agency and Iran to Congress. But those agreements do not appear to be in the offing.

On Thursday night, Schumer explained his opposition to the deal in a very long post for Medium, as most of the political world was focused on the first 2016 debate between the top ten Republican candidates. (Politico cited an anonymous source saying that Schumer intended to announce the news on Friday, but the White House leaked the news to the Huffington Post.)

White House allies pounced on Schumer immediately, questioning his fitness for leading the Democrats if he was so willing to blatantly cross Obama on a topic of such import.

Secretary of State John Kerry was a little more reserved, noting that “I profoundly disagree” with Schumer’s decision.

In his statement, Schumer explained that he just didn’t believe that Iran was acting in good faith.

“I believe Iran will not change, and under this agreement it will be able to achieve its dual goals of eliminating sanctions while ultimately retaining its nuclear and non-nuclear power,” Schumer wrote. “Better to keep U.S. sanctions in place, strengthen them, enforce secondary sanctions on other nations, and pursue the hard-trodden path of diplomacy once more, difficult as it may be.”