Virginia Senator Tim Kaine is among the most prominent Democratic supporters of the Corker-Menendez bill on Iran. He may be one of the most important players in determining whether it passes and what it ultimately looks like — which in turn could help influence whether a final Iran deal is reached and goes forward.

Critics fear Corker-Menendez could prematurely scuttle the whole negotiation process. The bill would suspend for 65 days President Obama’s authority to temporarily lift sanctions, pending a Congressional vote to approve or disapprove the final deal, and would require the president to certify Iranian compliance on several fronts, including whether Iran has directly supported an act of terrorism against Americans or American interests. Critics say this latter provision risks killing a deal later by introducing an element unrelated to Iran’s nuclear program.

Some Senate Dems will push amendments to change or eliminate the terrorism provision during the bill’s mark-up next week, but it’s unclear whether Republicans will acccept them. Critics also say a vote on the bill now could persuade Iranians that Congress will not allow the president to deliver on our end of the bargain later, perhaps upending sensitive negotiations.

I asked Kaine to respond to all of the criticisms. A lightly edited and condensed version of our conversation follows.

THE PLUM LINE: Isn’t it true that even if Corker-Menendez never passed, Congress could vote whenever it wanted to on the Iran deal after it were signed?

SENATOR TIM KAINE: The answer to that is Yes. Congress could put a resolution of approval or disapproval on the floor at any time.

PLUM LINE: If Congress can vote at any time to approve or disapprove the deal, with or without the existence of Corker-Menendez, why do we need Corker-Menendez in the first place? Since the primary function of Corker-Menendez would be to ensure a vote that would happen anyway, then why hold a vote on it now, when that could derail a deal before it is signed?

KAINE: The reason is this. Because a central aspect of this deal is what Iran must do to get out from under Congressional sanctions, Congress will be involved in approving the final deal. The question is, will we be involved under rules that are structured and deliberate and timely, or will we get involved under free-for-all rules? Corker-Menendez gives Congress rules that are defined in terms of procedures and timing, providing certainty that may even help the negotiators in the final phase of the negotiation.

PLUM LINE: The argument is that this essentially locks Congress into a process, rather than leaving it open-ended in a way that could result in a Congressional vote at any time, and on any particular set of conditions that Congress might dream up at any given moment? That this uncertainty could itself imperil the deal after it is signed?

KAINE: Yes, that is the argument. If what Iran wants is to be out from under the original sanctions, what is their incentive to really make major concessions if they have no idea about what Congress thinks or when Congress might act? Setting up a process with timing and structure will promote certainty.

PLUM LINE: Are you saying there’s no chance that a vote on Corker-Menendez before the deal is done could have the effect of derailing the process, by, say, empowering Iranian hard-liners to say, “See? Congress won’t allow the president to keep his end of the deal”?

KAINE: There is zero chance that Corker-Menendez passing will harm these negotiations. Iran is very sophisticated. They want out from under Congressional sanctions. They’ve known from the beginning that Congress would be involved in that. The question is, What is the process that Congress will use? The letter from the 47 Republicans is engagement under a free-for-all. It’s much better to have Congressional engagement under a standard that is agreed upon and timely, and I think this is deferential to the administration.

PLUM LINE: But let’s say Corker-Menendez passes. Is there anything stopping Congressional Republicans from reverting to that free-for-all later, if the final approval/disapproval vote under Corker-Menendez doesn’t go the way Republicans want?

KAINE: There’s no guarantee. But once you have a huge number of them voting for an orderly process, I think it becomes harder for them to revert to that free-for-all process.

PLUM LINE: Corker-Menendez has at least eight or nine Democratic co-sponsors. A veto-proof majority would appear within striking distance. Let’s say Congress overrode the veto. Would you anticipate that a number of those Democrats would still vote to approve the final deal anyway?

KAINE: If you posit a final deal that looks like the agreement announced Thursday? Absolutely. A number of us who will vote for Corker-Menendez are very supportive of diplomacy.

PLUM LINE: Part Two in the Corker-Menendez framework would be a vote to approve or disapprove the final deal. Can you explain what the various permutations are from that point?

KAINE: Let’s say Corker-Menendez passes. And let’s say there’s a final deal that looks like the framework. You’ll probably see in that 60-day review period discussions and expert testimony. You’d then likely see both resolutions of approval and disapproval of the final deal introduced.  The prospects of a resolution of approval passing both houses is tough. But a resolution of disapproval passing would be unlikely.

Either a resolution of approval or disapproval is subject to the 60-vote threshold. And if a resolution of disapproval passed, it would be vetoed by the president.  If he could convince one-third plus one in one house of Congress to stick with him on the veto, that amounts to “no action.” Which is then defined as “approval.” That’s a very deferential standard for the president.

PLUM LINE: So under the Corker framework, what Republicans will have accepted is this: If there aren’t 60 votes in the Senate for “disapproval,” or if there aren’t 67 to override it if there is a veto, the deal is essentially approved?

KAINE: The waiver of Congressional sanctions can begin. Yes. The high hurdle still remains the repeal of the sanctions statute. [This would make the lifting of sanctions, and the deal, permanent.] That would take an affirmative vote in both houses. But that would not likely happen for some time. Congress would want to wait, to test Iranian compliance.

PLUM LINE: The Corker bill also requires the president to certify every 90 days that Iran has not directly supported a terrorist attack against an American or American business, which could trigger a vote to reimpose sanctions. I understand you now have some problems with this provision?

KAINE: I think the provision is drawn pretty narrowly. But I do understand there will be colleagues of mine offering alternative amendments. I’m open to hearing them.

PLUM LINE: Is there going to be a genuine push to improve this? And if so, what would a better version look like?

KAINE: There’s been a change in the lay of the land since we introduced the bill. The [newly-agreed-upon] framework makes very plain that nothing in this deal will put any restrictions on the U.S.’s ability to do sanctions on non-nuclear activity, including terrorist activity. The clear statement that we will maintain full ability to do that may solve the terrorism concern.