Sen. Charles E. Schumer (D-N.Y.)  (Seth Wenig/AP)

While much of the political world (and the rest of America, for that matter) was enthralled by the first Republican presidential debate Thursday night, close watchers of Congress and foreign policy were just as intrigued by a piece of simultaneously breaking news: Sen. Charles E. Schumer (D-N.Y.) had come out against the Iran nuclear deal.

Why has a single senator's decision garnered so much attention? Because Schumer is the presumptive next Democratic leader of the Senate, in line to take the reins from Harry M. Reid (D-Nev.), when he retires after next year's elections, and thus, it is assumed, carries significant influence among Senate Democrats.

[Sen. Charles Schumer announces opposition to nuclear pact with Iran]

The decision generated immediate venom from liberal activists and from former aides to President Obama. MoveOn.org called it "outrageous and unacceptable that the Democrat who wants to be the party’s leader in the Senate is siding with the Republican partisans and neoconservative ideologues." Former Obama speechwriter Jon Favreau tweeted, "This is our next Senate leader?" -- to which former Obama aide Tommy Vietor added, "He just made that a lot less likely."

But for a variety of reasons, Schumer's decision is not that big a deal. It's not going to kill the Iran deal. It's not going to swing many, if any, Senate votes. And it's not going to keep Schumer from succeeding Reid as the Senate's top Democrat.

In the scenario with the most impact, it could mean the difference between an Iran disapproval resolution getting bottled up in the Senate and sending it to Obama for a veto that will ultimately stand.

Here's why:

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Arithmetic. While virtually every Democratic senator said initially that he or she was undecided on the Iran deal and planned to make a decision only after close study, only a small subset of the Democratic caucus is considered even close to likely to vote against it. Those include the eight senators who were early co-sponsors of the legislation establishing a congressional review process for the deal. Of those eight, two have announced support for the deal and only Schumer has declared opposition. (Another Democratic co-sponsor, Robert Menendez of New Jersey, is almost certain to be a no.) If no Republicans break ranks to support the deal, deal opponents would have to guarantee each of those undecided Democrats -- Joe Donnelly (Ind.), Heidi Heitkamp (N.D.), Michael F. Bennet (Colo.) and Richard Blumenthal (Conn.) -- maintained their opposition in order to block a Democratic filibuster, or try and recruit from the 12 Democrats who signed on to the Iran review bill only after the White House dropped its opposition. (One of those, Kirsten Gillibrand of New York, announced her support of the deal Thursday.)

Long story short: getting the 67 votes to override a veto would require a monumental feat of persuasion under the current circumstances, and getting the 60 necessary to block a filibuster is hardly assured.

Timing. Is Schumer capable of pulling off such a feat? Maybe: As his ascension to Democratic-leader-in-waiting shows, he's as masterful a navigator of internal Senate politics as anyone in his caucus. But anyone who thinks Schumer's decision will prompt a mass defection of Democrats has never had a close encounter with a senatorial ego. While some senators might find Schumer's reasoning compelling and might crib their own rationale from it, no senator will dare suggest they are voting against the Iran deal because Chuck Schumer is, too. And note that his decision came not in an impassioned floor speech, not in a private entreaty to his Democratic colleagues, not even in a YouTube video, but in an written statement posted online after the Senate has gone home for its month-long summer break. (A person close to Schumer says his intention was to make the announcement Friday -- a leak, the person said, forced the debate-time posting -- but still.)

Yes, the timing will allow well-funded deal opponents to tout Schumer's opposition in a month's worth of TV ads. But there is little sign thus far that Schumer himself intends to participate in a broader public relations campaign against the deal, whether by lobbying against it on Sunday talk shows or holding town hall meetings or participating in rallies during the recess. If Schumer were dead-set on killing the deal, he would have made his intentions known weeks, if not months, ago.

Politics. Schumer signaled in a statement Thursday that he does not plan to personally lobby Democrats against the deal: "There are some who believe that I can force my colleagues to vote my way. While I will certainly share my view and try to persuade them that the vote to disapprove is the right one, in my experience with matters of conscience and great consequence like this, each member ultimately comes to their own conclusion." And that is why -- Obama aides' outrage aside -- Schumer's standing in the Democratic caucus is not in doubt. Schumer's feelings about the deal are well known, and his opposition hardly registers as a surprise to anyone who has tracked his public statements on Iran's threats toward Israel, his longstanding alliances with pro-Israel advocates, or the feelings of his constituency.

Had he declared all-out war on Obama's foreign-policy centerpiece, perhaps it would embolden a challenger to his leadership ambitions. But the relationships Schumer has developed during his 16 years in the Senate run thicker than one vote. And by expressing his intent to treat this as a "matter of conscience," he is signaling that you can sometimes vote against the party line when you have to, which is an important message to send for a future leader of senators who will invariably find their own personal political considerations at odds with party loyalty from time to time.