During the first Republican debate, when everyone in politics was distracted, Sen. Chuck Schumer announced that he will vote against approving the Iran deal in the Senate. This is a big, big deal . Schumer will almost certainly be the next Democratic leader in the Senate, and is a hugely influential pro-Israel voice on the Hill. A small number of swing Democrats will decide the deal's fate, and Schumer's opinion could give them a green light to vote against the deal. This doesn't mean the Iran deal is dead, as it's not clear how many Democrats will follow Schumer. Moreover, Schumer's statement doesn't say he'd vote to override Obama's veto, which is the the vote that really matters in the Iran deal. But this is the one thing that could have put the Iran deal in actual jeopardy. Obama now has a fight on his hands.

Why Schumer matters

The Iran deal is supposed to lift American sanctions on the country in exchange for nuclear concessions. But Congress can vote to strip Obama's power to lift sanctions, causing America to break the deal's terms and theoretically collapse it. That's what Schumer just committed to doing.

"To me, the very real risk that Iran will not moderate and will, instead, use the agreement to pursue its nefarious goals is too great," he said in a statement published on Medium. "Therefore, I will vote to disapprove the agreement."

The reason this matters, in one word, is partisanship. The Iran deal is President Obama's top priority; he will be certain to veto any vote to reimpose sanctions. In order to overcome it, Republicans need votes from about a fifth of the Democrats in both the House and the Senate.

"The calculation still is we'll have the votes"

That'll be hard, given that top leaders like House Minority Leader Nancy Pelosi and Senate Minority Whip Dick Durbin support the deal. But Schumer's opposition — as heir apparent in the Senate and one of the leading pro-Israel voices in the party — gives Democrats real cover to oppose the deal. It's much harder to seem like a disloyal Democrat for voting against the deal if your leadership is also opposing it.

This doesn't mean Democrats will defect from the bill en masse. "The calculation still is we'll have the votes," Rep. Jan Schakowsky, a pro-deal Democrat, told the New York Times. It's still a big lift to oppose the president on his top foreign policy issue.

But before Schumer's announcement, there was no real fight. Now there is.

How far will Schumer go?

That said, Schumer's statement is very carefully worded. He doesn't ever say he'd vote to override an Obama veto — just that he'd vote "to disapprove the agreement." It could be that Schumer will vote against the deal to show his displeasure but vote to sustain Obama's veto on basic party-loyalty grounds.

There's no sugarcoating this for deal supporters

Moreover, the announcement during the Republican debate — don't laugh — matters. Schumer isn't dumb. He knows full well that the vast majority of the news media is watching TV right now, meaning his announcement will get way less media play than it would have otherwise. That suggests Schumer isn't trying to campaign against the bill, which would infuriate the White House even more, but is merely announcing personal opposition. If that's the case, then his influence in the party might be diminished.

But ultimately, there's no sugarcoating this for deal supporters: The most important person in Congress on the Iran deal just announced his opposition to it. It's easily the biggest setback the Iran deal has faced so far — and there's a real risk it's a deadly one.