At this point, the biggest practical impediment to moving from Obama's Iran framework to a formal deal is that the US Congress might scuttle the arrangement. Chuck Schumer raised the odds that scuttling will happen by reiterating his support for a bill written by Bob Corker (R-TN) that would give Congress the right to approve or reject any relaxation of sanctions on Iran. Schumer's support for Corker bill isn't new, but some Democrats who had previously backed the bill had indicated wavering support once a deal was in hand. Schumer's support is particularly relevant because he is set to succeed Harry Reid as the Democratic Party leader in the US Senate. That should undermine White House efforts to paint opposition to Corker as a matter of partisan loyalty.

Chuck Schumer is a really sincere and committed Israel hawk

As it happens, I was an intern in Schumer's New York office 15 years ago. Many politicians on Capitol Hill do not have deep convictions about questions related to Israel, and largely let their actions be dictated by political considerations — which, given the clout of pro-Israel lobbying groups, tends to push them in a more hawkish direction.

Fifteen years ago, at least, Schumer was very much the opposite kind of politician.

I spent some time with him before and after press events on Israel-related matters, and the issue was both a genuine passion point for him and something where the communications staff tended to try to sand the hawkish edges off his gut reactions to events.

In 2010, Schumer explained to a New York radio station that his name "comes from the word shomer, guardian, watcher" and that he believes that "one of my roles, very important in the United States Senate, is to be a shomer — or the shomer Yisrael. And I will continue to be that with every bone in my body."

In other words, if Schumer thinks scuttling a deal is the right thing for Israel, then he is going to work hard to scuttle the deal.

Can Obama uphold a veto on Corker's bill?

Of course, just because Bob Corker may be able to pass a bill doesn't mean the bill will become law. The real question is whether hawks in Congress can muster enough votes to override an Obama veto. To accomplish that, Republicans need a two-thirds supermajority in both houses of Congress.

Thanks to the filibuster, we've grown accustomed to the Senate as being the place where majority-supported legislation goes to die.

But given Schumer's support for the Corker bill, the harder road for the GOP may be through the House of Representatives. It will take 290 House votes to overcome a veto in the House. Assuming the GOP can count on all 245 Republicans, that still leaves them in need of 45 Democrats — and due to a mix of gerrymandering and Democrats' poor performance in the 2014 midterms, there are very few Democratic incumbents holding Republican-leading seats.

Steny Hoyer could be the key player

Republican hopes, then, depend on finding Schumer-like figures — Democrats in position of authority in the caucus who support a hard line on Iran out of conviction rather than political expediency. A pivotal player here is likely going to be Steny Hoyer, the number-two Democrat in the House and traditionally a strong ally of pro-Israel groups. On April 2, Hoyer offered a broadly supportive but also noncommittal statement on the Iran framework indicating that he is interested in listening to more arguments.

If Hoyer stays on-side, then it is hard to see how the bill could overcome an Obama veto regardless of what happens in the Senate. But if Hoyer joins Schumer as another prominent caucus leader bucking the president, the deal could be in real trouble on Capitol Hill.