The media is already running tallies of how the final Congressional vote on the Iran agreement is likely to go. The assumption is that both houses will pass a "Resolution of Disapproval" first. President Obama will veto it. And the ultimate vote will come when Congress attempts to override Obama's veto, thus killing it once and for all.
To override requires a two-thirds vote in both houses. Because the Republicans do not constitute two-thirds of either house, a successful veto override will require (assuming every Republican votes against the President) support from 15 Democratic senators plus 43 Democratic House members.
That is a total of 58 Democrats who must vote with the Republicans against an agreement negotiated by a Democratic president.
That will not happen. It won't happen (beyond the merits of the deal) because Democrats will not want to go into their 2016 election campaigns having just repudiated the Democratic president and their 2016 nominee. All the Democratic candidates for the 2016 nomination support the agreement and former Secretary of State Hillary Clinton even claims credit for starting the process that produced it.
Any Democrat who votes against the agreement is saying that the single biggest foreign policy achievement of the incumbent Democratic administration is a dangerous failure. Specifically he will be buying into the key arguments against the deal -- that it will deliver (rather than prevent) a nuclear Iran, thereby jeopardizing American national security and, in the words of Sen. Lindsey Graham, issuing a "death sentence" to Israel.
Imagine the GOP ads with those messages!
Republicans will run ads like that against Democrats no matter how they vote on the deal (after all, they are still Democrats). Prevailing against them requires fighting back, demonstrating that the agreement enhances both U.S. and Israeli security. Opposing the agreement is acknowledging that the Democratic administration knowingly jeopardized the security of the American people and the existence of the State of Israel.
Following that admission, how could any Democrat ask for support for himself, let alone for the Democratic nominee for president? For the party that sold out both the U.S. and Israel to Iran!
In other words, voting against the Iran deal is not just wrong, it is stupid politically. And that, more than anything else, is the reason the agreement will pass. Enjoy the sideshow but that is all it is. The Iran agreement is a done deal.