French Foreign Minister Laurent Fabius, second right, German Foreign Minister Frank-Walter Steinmeier, second left, British Foreign Secretary Philip Hammond, left, U.S. Secretary of State John Kerry, center, and Austria’s Foreign Minister Sebastian Kurz, right, talk in Vienna, Austria last month when the deal was announced. (Joe Klamar/Pool Photo via AP)

As more and more undeclared Democratic senators announce their support for the Iran deal, it looks increasingly likely that the nuclear agreement will survive its congressional trial – even opponents are starting to accept that seeming inevitability.

Which leaves just one question: Who will be the deal-clinching senator No. 34?

Congress loves drama and on big legislative ventures, being that last senator to make or break a bill’s  fate can bring political rewards – or put a target on your back.

“In legislative politics, the idea for some is to get yourself into that situation where you are the last vote in one direction or another, and that allows you to extract the most rewards from both sides,” said Christopher Arterton, a professor of political management at George Washington University, pointing out that senators by holding out on committing their vote can stand to gain political clout, committee assignments and promises from party leaders to pay attention to state projects.

In the past, the magic number in the Senate has been 60, the number needed to prevent a filibuster. On the Iran deal, the number is 34 because Congress will be voting on a resolution of disapproval – where a “yea” vote means the senator opposes the deal — and that many votes would ensure Congress can not override the president’s promised veto delivering a victory to the White House.

Lawmakers have had more than a month to review the details of the Iran deal, and though the undecideds say they are still deliberating, many congressional experts and lobbyists who have been working this issue agree that enough time has passed for senators to have made up their minds. But so far, only 29 Democratic senators have publicly backed the deal, in which Iran promises to limit its nuclear program in exchange for an easing of economic sanctions, while two have promised to vote against it.

For the rest, their silence may be less due to a state of true protracted indecision as simply trying to get the timing right and determining if the attention that comes with being the decisive vote brings rewards or too many risks.

Recent polls suggest that across the country, and especially in key swing states, more people oppose the deal than support it. While there seems to be a significant party split in those numbers, they give Republican opponents of the deal more fodder to use the Iran vote as a campaign issue in 2016. If implementation of the deal doesn’t go well, it could be an issue in future elections too.

“I will take an educated guess that Bennet does not want to be No. 34,” said Jennifer Duffy, senior editor with the Cook Political Report, referring to the Colorado Democratic Sen. Michael Bennet who is up for reelection in 2016 – and is still undecided on Iran. “But for all these guys that are supporting it, the proof for them will be in how it goes… what seems like a smart position today may not be a smart position in a few years.”

The undecided Senate Democrats represent all walks of political life.

There are members who are up for reelection, like Bennet; members who have received a good deal of financial support from donors openly opposed to the deal, like Cory Booker (D-N.J.); Jewish members who have been getting significant pressure to vote against the deal from Israel and aligned groups, like Ben Cardin (D-Md.);  and members with aspirations to take on leadership roles, either in the realm of foreign policy or generally within the Democratic party, like Cardin, Chris Coons (D-Del.) and Mark Warner (D-Va.).

These senators also face the question of whether the attention the vote would get in Washington would matter much back home.

Becoming Senator No. 34 means that “certainly you’ll get a lot of love from the White House,” said Duffy. “But I’m not sure your constituents are going to give you a lot of love.”

There remains a possibility that enough senators will back the deal so that the resolution can be successfully filibustered, sparing the president the need to issue a veto.

In this case the attention will turn to senator number 41.

“There is something about these announcements being made – from what we can tell, they’re going to continue and most of the members will be committed ahead of time,” said Thomas Mann, a congressional expert with the Brookings Institution. By the time the vote comes around next month, he continued, “there’s a good chance we’ll be talking about the filibuster and not the veto in the Senate, if and when they get to 34 with a good number of remaining senators yet to commit.”

By The Post’s count, the Senate is very close to the 34 mark already, if you include senators who appear to be leaning pro-deal, and only about a dozen are still truly undecided.

[Whip count: Obama on the cusp of victory for Iran deal]

No one element of the deal is likely to convince all the senators still on the bench to throw their support behind it and no one incentive is likely to make them race to the nearest microphone to make an announcement.

Senators have portrayed their decision on whether to support the Iran deal as a vote of conscience. If the benefits or drawbacks of when they reveal their decision is also a factor, don’t expect them to talk much about that.

“They certainly will have an explanation that goes to the perceived merits of the case,” Arterton said. “It’s not something that you’re going to be able to tell conclusively.”