One hundred and fifty House Democrats have now signed a letter expressing strong support for President Obama’s ongoing negotiations with Iran over its nuclear program, I’ve learned, improving the chances that an eventual nuclear deal could survive the Congressional oversight process.

The 150 Democratic signatories — which include Dem leaders like Nancy Pelosi and Chris Van Hollen — are significant. Here’s why: If a deal is reached that looks like the recently-announced framework, and the GOP-controlled Congress votes to disapprove of it, it’s now more likely that there will be enough House Democrats to sustain Obama’s veto of that disapproval legislation, allowing the deal to move forward.

Under the current Corker-Cardin framework establishing oversight of the Iran deal, Congress would vote to approve or disapprove of a final deal, determining whether Obama has the authority to temporarily lift Iran sanctions to implement it. If Congress fails to pass disapproval of the deal — or if Congress does pass a disapproval measure, then fails to override Obama’s veto of it — the deal moves forward. Of the 150 House Dems who signed the letter, 145 are voting members — just over the number that would be necessary to sustain a veto.

The letter, which was spearheaded by Dem Reps. Jan Schakowsky, Lloyd Doggett, and David Price, says:

As negotiations over Iran’s nuclear program continue, we urge you to stay on course, building on the recently announced political framework and continuing to work toward a strong and verifiable agreement between the P5+1 countries and Iran that will prevent Iran from having a nuclear weapon….We must exhaust every avenue toward a verifiable, enforceable diplomatic solution in order to prevent a nuclear-armed Iran. If the United States were to abandon negotiations or cause their collapse, not only would we fail to peacefully prevent a nuclear armed Iran, we would make that outcome more likely…
We must pursue diplomatic means to their fullest and allow the negotiations to run their course — especially now that the parties have announced a strong framework — and continue working to craft a robust and verifiable Joint Comprehensive Plan of Action by June 30. We must allow our negotiating team the space and time necessary to build on the progress made in the political framework and turn it into a long-term, verifiable agreement….We look forward to continuing our shared work on this important matter.

The letter does not commit its signatories to ultimately supporting a final deal. But, in declaring support for seeing negotiations through — and in particular for turning the “strong” framework into a long term deal — it suggests that its signatories will likely support such a deal if it does look like the framework the parties already agreed upon (which is obviously far from a certain outcome).

“This letter reflects wide support for the agreement as outlined in the framework,” Rep. Schakowsky told me. “If there is a vote in the Congress to disapprove of the final agreement, and it is an agreement that reflects the framework, then I think we have enough votes in the Congress to sustain a veto.”

The Corker-Cardin bill providing a framework for a vote on whatever emerges from the Iran talks has not yet passed the Senate. But now that Mitch McConnell has moved to cut off amendments that could kill the bill, it is likely to ultimately become law, requiring a final approval-or-disapproval vote on any final deal. Republicans are likely to vote to disapprove of the final deal, if there is one, which would then be vetoed by the President. He would then need just one-third of one chamber to stick with him on the veto for the deal to proceed, at least in the near term.

But the goal of the letter isn’t just to signal that a veto would likely be sustained. It’s also meant to signal that there is more Congressional support than expected for the negotiations in order to help along the talks.

“Having such Congressional support should be helpful in these final negotiations,” Doggett says. “We believe this gives the president the ability to negotiate an agreement without concern that he will be undermined in the House.”

“The significance of the number of names on the letter is that we have a critical mass in the House that is strongly supportive of seeing the negotiations through, and doesn’t want to see Congress derail or damage that process,” Rep. Price says. He adds that Dems also hope to win over some Republicans: “I have to believe that there are fair-minded Republicans who would take an objective look at the deal.”

Daryl Kimball, the executive director of the Arms Control Association, thinks the letter could also help by giving negotiators leverage in the final talks. “This will strengthen the hand of U.S. and P5+1 negotiators in the final rounds of difficult negotiations with Iran, because it reinforces the fact that the Obama administration will likely have sufficient political support from Congress to follow through on the U.S.’s commitments in the framework agreement,” Kimball says. “This expression of Congressional support for the framework should make it more likely that the negotiators can finalize the remaining details before June 30.”

Obviously there is still a very long way to go. A deal might not be reached at all. Or a final deal might be a bad one that causes some Democrats to defect. Depending on what the deal looks like, you could very well see a close vote in the House that falls short of sustaining an Obama veto. (It’s harder to imagine enough Dems in the Senate siding with Republicans to override a veto, however.)

Still, Democratic aides say that a handful of additional House Dems signaled support for the negotiations without signing the letter, which could give them a bit of breathing room. The bottom line is that, while it is anybody’s guess what will ultimately happen with the Iran talks and subsequent Congressional votes on a final deal, this new letter can only help.