Today, three Democratic Senators came out for the Iran deal: Tim Kaine of Virginia, Bill Nelson of Florida, and Barbara Boxer of California. The math now looks increasingly difficult for the deal’s opponents.

Kaine and Nelson are both quite significant. They are both genuinely moderate; previously, the only ones who had come out for the deal were liberals or were always all but certain to do so: Dianne Feinstein; Martin Heinrich, Tom Udall, and Dick Durbin, who is rounding up support for the accord inside the Dem caucus.

What’s more, both Kaine and Nelson were early supporters of the original bill creating an oversight mechanism for Congress on the Iran deal, a group that was seen as skeptical early on. Kaine subsequently played a lead role in brokering the final compromise on that oversight bill. He’s widely seen as knowledgeable on foreign policy. He’s a serious vice presidential contender from a major swing state. So while most people thought he’d probably back the deal in the end, having it official represents a boost to the deal’s chances.

Taken together, Nelson and Kaine are key because they reduce the size of the pool of Dems thought to be gettable by the opposition. The group of around eight or nine Democrats who backed the original oversight bill have long been seen as the most likely to oppose the accord. In addition to Nelson and Kaine, those include: Chuck Schumer, Heidi Heitkamp, Richard Blumenthal, Joe Donnelly, Michael Bennet, Robert Menendez, and Angus King (an independent who caucuses with Dems).

Take Nelson and Kaine out of that pool, and you’re left with around seven Senate Dems who seem like they could genuinely still vote No. Seven others who are thought to be undecided, or at least who can’t be ruled out as No votes: Harry Reid, Chris Coons, Benjamin Cardin, Joe Manchin, Cory Booker, Kirsten Gillibrand, and Tom Carper. If all of them vote No, that’s 14 Senate Dems opposing the deal. Opponents need 13 in order to get 67 Senators to override Obama’s veto of a measure blocking the accord.

So opponents need to basically run the table, getting all but one of those 14 Senators. Carper is now leaning towards the deal. So is Joe Manchin. That is going to be very hard to pull off.

To be sure, perhaps opponents can turn the tide during the August recess with tens of millions in ads. And there’s no question that if Chuck Schumer opposes the deal, the media explosion will be deafening.

But even if Schumer does that, the real tell will be in how vociferously he opposes the deal and, even more important, whether he prevails on fellow Dems to join him. (If you don’t believe me, you just wait to see what kind of pressure opponents put on him to do just this.) It’s very hard to imagine the incoming Senate Democratic leader actively helping Republicans to sink Obama’s signature foreign policy achievement. He could easily end up opposing the deal while enough Dems support it to sustain Obama’s veto.

And right now the numbers to sustain the veto increasingly appear to be there.

****************************************

UPDATE: A source in favor of the deal gets in touch to argue that three other Senate Dems might be gettable by opponents: Gary Peters, Bob Casey, and Ron Wyden. So perhaps the pool of undecideds is slightly larger than I indicated.

But still, Kaine and Nelson signal that the momentum is going in the right direction. And opponents still need to shift a very large number of Senators in their direction.

UPDATE II: Senator Robert Menendez’s office gets in touch to clarify that he has not come out officially against the Iran deal (though he has serious concerns about it), as I previously suggested. I’ve edited the above for accuracy.