President Obama’s magic number on the Iran nuclear deal is 41.

Senate Republicans need 60 votes to break a Democratic filibuster, but they’ve only won over two Democrats so far. If 41 senators vote against a GOP resolution disapproving the deal next month, Obama will earn a big legislative and foreign policy victory.

The White House isn’t there yet, but it is inching closer to a victory.

Sen. Patty Murray (D-Wash.) on Tuesday became the 29th Senate Democrat and Independent to publicly back the agreement, according to The Hill’s whip list. Only two Democrats oppose the deal, leaving 15 undecided Democrats.

Groups on both sides of the issue have blanketed airwaves over the August recess, urging lawmakers to support or oppose the deal.

That puts an enormous amount of pressure on the few undecided votes left in the Senate. Many lawmakers facing angry constituents on both sides of the issue might, at a minimum, want to avoid being seen as the deciding vote that put the Iran deal on top.

Such votes can be used in countless future political ads. And while there is safety in numbers, there’s no certainty. A number of Democrats, some of whom lost reelection bids, have been attacked as the deciding vote on ObamaCare, for example.

Here’s a look at the Democrats that might face a tough decision before the September 17 deadline for a vote on Iran.

Sen. Ben Cardin (D-Md.)

The ranking member of the Foreign Relations Committee is the most influential lawmaker still on the fence on the Iran nuclear deal.

Sue Walitsky, Cardin’s spokesperson, said that he has speaking with Marylanders during the August recess, adding that “emotions have been intense on all sides of the debate, but the feedback has been decidedly mixed.”

Cardin’s state has been blanketed with ads from groups on both sides of the deal, and the Maryland Democrat was the target of the first Snapchat ad in the Iran debate from Secure America Now, which opposes the deal.

If he decides to back the deal he could sway other on-the-fence Democrat to support the agreement as well, but Walitsky said that Cardin “does not plan to rush his decision based on what others may decide.”

Cardin is expected to remain on the fence until after Labor Day, when lawmakers return to Washington from the August recess.

He’s not the only undecided Maryland lawmaker. Sen. Barbara Mikulski is also on the fence. Asked about a potential timeline and any feedback she’s received, a spokesperson pointed back to a July statement.

“Congress must determine if this deal has the robust verification levels needed to ensure Iranian compliance,” she said at the time. “A good deal must block all pathways to a bomb.”

Unlike the other 14 undecided Democrats, Mikulski is retiring at the end of her current term. That means she could, if she decides to support the deal, hand Obama a political victory without having to worry about her next election.

Sen. Richard Blumenthal (D-Conn.)

Blumenthal is one of four undecided Democrats who are running for reelection next year and groups on both sides of the Iran nuclear deal are running ads in Connecticut in an effort to sway his vote.

Blumenthal, who has been careful to not tip his hand, said earlier this month in Connecticut that there are “weaknesses” in the agreement.

“The question is whether those weaknesses justify voting against the agreement. And the key question is what will happen if Congress rejects [the deal.] What are the alternatives?” he added, according to CNSNews.com, which uploaded a video of the event.

A spokesperson for Blumenthal said that the senator is still undecided with no timeline for a decision, but the Connecticut Democrat told the Associated Press that he would likely announce a decision “sometime in the next couple of weeks.”

That could put his announcement around Labor Day, shortly before Congress is expected to return to Washington and take up the Iran deal.

Sen. Tom Carper (D-Del.)

The Delaware Democrat is still officially undecided, but says he’ll likely back the agreement.

“I’m leaning yes, but I’m going to hold off until I actually finish reading the deal over the next week or so,” Carper told Delaware Public Media earlier this month, adding that he’s “doing my homework [and] doing my due diligence.”

He added that he expects to have his decision “within two weeks." Under the senator’s timeline, his announcement on the Iran deal should be imminent.

Sen. Chris Coons (D-Del.)

It’s less certain how Carper’s Delaware colleague, Democratic Sen. Chris Coons will vote.

He told WBOC, a local Delaware TV station, last week that his “initial response to the deal was real disappointment.”

“I do think that the vote margin here is going to be very close. It is, I think, likely that the Senate will vote to move forward a resolution of disapproval...whether or not there are the votes to override the president’s veto I think will be a very close call,” he added.

Coons, a member of the Foreign Relations Committee, said he will likely announce his position before the Senate votes, but didn’t give a specific timeline for when that could be.

He said, in a separate interview with WMDT, that he’s been hearing from both sides of the deal, including Delawareans who want him to support the agreement and the pro-Israel community, which wants the administration to negotiate a better deal.

Sen. Jeff Merkley (D-Ore.)

Merkley is one of four Senate Democrats considered as likely to support the agreement, according to The Hill’s whip list.

The Oregon Democrat has been largely silent on the deal since it was announced, but said in a July statement that the agreement “is a significant milestone in the effort to preclude Iran from pursuing a nuclear weapon.”

He added that the agreement must block Iran’s path to getting a nuclear weapon.

Sen. Gary Peters (D-Mich.)

The Senate’s only freshman Democrat is under increasing pressure after his colleage Sen. Debbie Stabenow (D-Mich.) and two Michigan House Democrats endorsed the Iran nuclear deal on Monday.

Liberal outside groups are specifically targeting the Michigan Democrat, with Zack Malitz, the campaign manager of CREDO Action, saying that “all eyes” are on Peters.

“Progressives want to know whether he will vote for peace or vote for war,” Malitz added.

But Peters, whose office didn’t respond to request for comment, told Interlochen Public Radio, a local NPR affiliate in Michigan, that he’s “still weighing all of the issues” with lawmakers having until roughly Sept. 8 to make a decision.

“This will probably be one of the serious votes I will make, no matter how long I’m in the United States Senate,” he added, mentioning that he’s spoken directly to President Obama and Secretary of State John Kerry about the deal.

Sen. Mark Warner (D-Va.)

The purple-state Democrat is still on the fence over the Iran deal, even though his Virginia colleague, Sen. Tim Kaine, announced his support before the August recess.

A spokesperson for Warner said that the senator is still reviewing the deal and that “I don’t think a decision is imminent.”

Warner told WINA, a local radio station in Virginia, that “the question that I’m wrestling with is, you know, it’s easy to Monday morning quarterback any deal, but on a going forward basis if we reject this deal are we really going to be maintain the sanctions regime… when the rest of the world has in effect said ‘no we think this deal is okay?'”

Other Senate Democrats have pointed to the dismantling of international sanctions even if Congress rejects the deal as key reasons they’ve decided to support the agreement.

Red-state Democratic Sens. Michael Bennet (Colo), Heidi Heitkamp (N.D.) and Joe Manchin (W.Va.) are also undecided. A spokesperson for Bennet, who faces a challenging reelection bid, didn’t respond to a request for comment on a potential timeline for the Colorado senator.

Meanwhile, Manchin is expected to support the agreement, but he’s also up for reelection in 2018 in a state where Obama is deeply unpopular making it politically risky to be senator that hands Obama a victory on Iran.

Sen. Cory Booker (D-N.J.)

Booker is under enormous pressure in New Jersey to oppose the deal.

But he also is seen as having presidential ambitions of his own, which could make it tough for him to oppose the liberal groups backing the accord. Democratic Sens. Kirsten Gillibrand (N.Y.) and Amy Klobuchar (Minn.), for example, are both supporting the deal.

Rabbi Shmuley Boteach, a friend of Booker, and New Jersey Gov. Chris Christie, a Republican, held a press conference on Tuesday against the Iran deal in the latest example of the intense lobbying effort aimed at Booker.

Fellow New Jersey Sen. Robert Menendez is one of only two Senate Democrats who are opposing the deal.

Ben Loehrke, a senior policy analyst for the Washington-based Ploughshares Fund, said it could be “difficult” for Booker to vote against his delegation, adding that “the fact that Senator Booker has taken this long to make his position known, it tells me that he’s really weighing [the deal].”

For the time being, Booker is staying on the fence, saying in a statement late last week that “I want to hear all angles on the deal and deeply weigh its short and long-term implications before making a decision.”

He added that he is holding the Iran deal to a “very high standard.”

This article was written by Jordain Carney for The Hill on Aug. 26, 2015.