Will Obama’s Iran firewall hold?

President Obama is counting on House Democrats to be his last line of defense against congressional opponents of the nuclear deal with Iran.

Many key players remain undecided, however, and some lawmakers are speculating that as much as half the House Democratic Caucus might be willing to buck the White House.

“If [Republicans] bring before us a resolution that says ‘We hereby approve the deal,’ I think that as many as half the Democrats would vote against it,” Rep. Brad Sherman (D-Calif.), a critic of the Iran deal, said this week.

{mosads}Facing unified Republican opposition to the deal, the White House is working hard to get House Democrats on board.

Top officials from Obama on down have been meeting personally with Democratic lawmakers in recent days, with the president even bringing some of them into the Situation Room to go over classified details.

At some point in September, Congress is expected to vote on the Iran deal. A vote against the pact would prevent Obama from lifting congressional sanctions on Iran, potentially blowing up the agreement.

Should a resolution against the deal pass, Obama would almost certainly veto it, setting up a final showdown in Congress.

If Republicans were to override Obama in the Senate — no small task, since it would likely require at least 13 Democratic votes — the fight could come down to House Democrats.

Assuming House Republicans vote in a unified bloc, Democrats could afford to lose no more than 43 members to sustain an Obama veto.

But corralling 145 lawmakers to back the president on Iran won’t be easy, given the likelihood of divisive vote that many lawmakers say will be among the most consequential of their careers.

“I’ve got 60 days,” Rep. Eliot Engel (N.Y.) — the top Democrat on the House Foreign Affairs Committee — said on his way out of a closed-door briefing on Capitol Hill this week, when asked about whether he backs the deal.

“I’m listening, learning and reading, and I have time to make a decision.”

After a Thursday morning meeting with Obama in the White House Situation Room, Rep. Adam Schiff (Calif.) — the top Democrat on the Intelligence Committee and another key undecided vote — told MSNBC that Obama is “well into the weeds on this issue” and appears intent on making the case to Democrats.

“He really, I think, feels very positive about this deal and was ready to make the case to us,” Schiff said. “He’s quite unequivocal about where he thinks the merits of the agreement are.”

The crucial role of the House Democrats in the Iran debate was apparent last week, when Vice President Biden trekked to Capitol Hill to meet with them mere hours after the deal was struck. Biden gave a detailed briefing on the diplomatic accord, which aims to limit Iran’s ability to obtain a nuclear bomb in exchange for the reduction of global sanctions on its oil and financial sectors.

This week, Democrats appeared to be the main target of a closed-door briefing for the full House with Secretary of State John Kerry, Energy Secretary Ernest Moniz and Treasury Secretary Jack Lew. Kerry and Moniz followed up the next day with a closed-door meeting for Democrats only.

The sales job has even gone international.

Obama is halfway through a six-day trip to Africa. At various points during the transcontinental journey, no fewer than 17 House Democrats will join him on Air Force One — a prime opportunity for him to talk up the Iran accord face-to-face.

It’s clear that many lawmakers have deep reservations about the deal.

During the closed-door briefing with Kerry and Moniz on Thursday, Democrats brought up concerns that “after 15 years, Iran will be a nuclear threshold power,” Engel said.

“In many ways this doesn’t prevent Iran from becoming a nuclear power… it just postpones it,” he added. “That’s my major concern.”

Sherman said Democrats inclined to vote against the Iran deal might choose not to override a veto, if it comes to that. 

“I could even see Democrats voting differently on passage of a resolution of disapproval and on overriding a veto of that,” Sherman said. “Because the first vote would be a symbolic vote — ‘do we approve of the deal?’ — the second vote really puts Congress at war with the president.”

Rep. Jan Schakowsky (D-Ill.), a supporter of the deal, brushed off speculation that many Democrats would break rank. 

“I feel very optimistic that we can sustain a veto,” she said. “I say that as someone who has talked to a lot of members and collecting names.”

“I feel optimistic that we will have enough.”

Supporters are going to have a bevy of outside supporters to help them reach the magic number.

Liberal activist groups such as MoveOn, Credo and Democracy for America are mobilizing behind the deal.

J Street — a pro-Obama Israel lobbying group — has raised $2 million in the last two weeks that it says will go towards TV advertisements endorsing the pact.

Those supporters are up against critics of the deal such as AIPAC, which have deeper pockets and are rallying to kill the accord with equal fervor.

Liberal groups acknowledged they have their work cut out for them during the long August recess, when lawmakers will be away from Washington and weighing how to vote on the deal.

“You’re going to see activism in Washington, you’re going to see grassroots activism, you’re going to see very visible communication with members of Congress,” said Zack Malitz, the campaign manager at CREDO Action.

Democrats also have their future to think about. Their vote agreement could haunt them in next election — and throughout the course of their career.

“This is like an Iraq war moment,” said Nick Berning, communications director at MoveOn.

“That’s a vote that our members still haven’t forgotten.”