After a big victory on the Iran nuclear deal this week, the Obama administration is looking to lock down support from the handful of still-undecided Senate Democrats.

The White House has gained enough votes to ensure the agreement ultimately survives Congress. But it's still at least three senators short of the 41 needed to prevent a resolution of disapproval from passing in the first place.

The president can only afford to lose two of the remaining five undecided Democrats if he wants to avoid having to issue a veto — an action he's taken only four times during his presidency.

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But some of those still on the fence have been skeptical, if not openly critical, of the agreement, underscoring the administration's tough sell in the final days of a months-long debate.

Sen. Ben Cardin (D-Md.), the ranking member of the Foreign Relations Committee, on Friday became the third Senate Democrat to say he will oppose the deal, joining Sens. Charles Schumer (D-N.Y.) and Robert Menendez (D-N.J.).

But the blue-state Democrat, who isn't up for reelection until 2018, is widely expected to support the agreement. Cantwell has previously distanced herself from legislation that would increase sanctions against Iran.

That leaves four Senate Democrats who could make or break the administration's push to avoid using a veto:

Blumenthal, the only holdout from his state delegation, is facing a mountain of pressure from both sides.

Conservative talk show host Larry Kudlow has threatened to run against Blumenthal in 2016 if he comes out in support of the Iran agreement.

Meanwhile, outside groups on both sides are running ads in the state aimed at securing Blumenthal’s vote. Members of MoveOn.org spoke with the senator at his office in Hartford late last month.

"He's very straightforward that he's talking to everybody,” said Tom Swan, who is working with MoveOn in Connecticut and was at the meeting. He added that “I would be shocked and very disappointed if he didn't end up doing the right thing.”

Blumenthal told The Hill earlier this week that while there are “number of downsides” to the agreement, “the question for the nation is whether there’s a better agreement or an alternative to this one.”

He previously backed legislation earlier this year that would have increased sanctions against Iran if the negotiators failed to reach an agreement by the June 30 deadline.

Jamal Abdi, the executive director of the pro-deal NIAC Action, said that while Blumenthal is “definitely one of the most hawkish Democrats on Iran,” he could ultimately support the deal.

Though fellow Michigander Sen. Debbie Stabenow (D-Mich.) endorsed the deal nearly two weeks ago, Peters remains undecided.

Unlike his predecessor, Sen. Carl Levin (D-Mich.), Peters has taken a hawkish stance on Iran, backing additional sanctions against the country even before he was sworn into office.

Peter suggested during an interview late last month with Interlochen Public Radio that parts of the nuclear agreement make him nervous.

“Iran wants to enrich uranium. ... Enrichment can lead down the path to building a weapon,” he added. “There’s no reason for them to enrich for them to have a peaceful program, and that part is troubling.”

The U.S.-Iran relationship is also personal for Peters, who has a constituent currently being held in the country.

But he's also feeling pressure from progressive groups.

Matt Blizek, who has been overseeing MoveOn’s grassroots mobilization on the Iran deal, said that in the past week, supporters have made roughly 1,000 calls in Michigan, aiming to sway Peters to support the deal.

While Peters was initially viewed as a likely “no” vote on the agreement, Abdi added that as the Senate’s only freshman Democrat, “it would be a pretty extraordinary move” for him to buck Obama on a key foreign policy issue.

“I think he [also] might be a little boxed in by the fact that most of his colleagues in his delegation ... are all for the deal,” he added.

While Manchin said shortly after the deal was announced that it would be “would be a catastrophe to walk away,” he’s facing a tough wall of opposition to the deal back in his conservative home state.

But even if he does end up siding with Obama, he may not vote to help the president avoid using a veto.

Manchin suggested Thursday during a town hall meeting on Iran in Charleston, W.Va., that he wouldn’t support a Democratic filibuster of the Iran resolution.

“I just think it’s too important to circumnavigate that thing because of political posturing, and I won’t do that,” he said, according to the Charleston Gazette-Mail

Manchin might not be alone. Sen. Chris Coons (D-Del.), who backs the deal, told CNN that it would be "regrettable" if Congress didn't vote on the agreement.

Progressive groups are expected to keep up the pressure on Manchin, though, with MoveOn launching online ads against him next week as part of a six-figure ad buy.

Abdi suggested that of the Democratic undecideds, Wyden — a Jewish lawmaker who is up for reelection next year — is the most likely to vote against the deal.

“I am surprised he is somebody who has been leaning ‘no’ on this,” he added. “He did not clash with the president over the sanctions. ... I don't know where this change heart comes from.”

Like his undecided colleagues, he’s facing pressure from both sides. Liberal activists have demonstrated outside of his offices in Oregon, while the American Security Initiative — a group that opposes the deal — targeted Wyden with an ad and asked its supporters urge him to reject the agreement.

The Oregon Democrat, who has described himself as “skeptical,” has also dismissed a key argument of those who favor the deal: that lawmakers are facing a choice between supporting the agreement and cheerleading a possible military conflict with Iran.