President Obama is facing a dilemma over legislation allowing the families of 9/11 victims to sue Saudi Arabia in U.S. courts.

The House rallied behind the legislation on Friday, passing it by unanimous voice vote. The Senate had already passed the same legislation unanimously.

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Obama has fiercely opposed the bill, arguing it could both strain relations with Saudi Arabia and also lead to retaliatory legislation overseas against U.S. citizens. The Saudi government has led a quiet campaign in Washington to kill the legislation.

Those efforts have been fruitless in Congress, however.

The legislation has broad support from both parties, and Congress could override an Obama veto for the first time if he rejects the legislation.

Such an outcome would undoubtedly embarrass Obama and divide Democrats ahead of the 2016 elections and a crucial lame-duck session of Congress.

Friday’s vote in the House was heavy with symbolism, taking place on the eve of the 15th anniversary of Sept. 11 terrorist attacks.

“I think the pressure is the vote,” said Rep. Jerry Nadler (D-N.Y.), an original backer of the bill and one of its 29 Democratic co-sponsors.

Lawmakers on Capitol Hill are unsure whether Obama will actually use his veto pen on the Justice Against Sponsors of Terrorism Act.

“I presume they would have to think very carefully about a veto because it might very well be overridden,” said Nadler.

To override the president, supporters would need a two-thirds majority in each chamber.

“I think the votes will be there to override it,” said Rep. Pete King (R-N.Y.), who introduced the bill in the House.

The White House is clearly aware of the dicey political waters in which it is sitting.

Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE supports the bill, as does Sen. Charles Schumer Chuck SchumerJacobin editor: Primarying Schumer would force him to fight Trump's SCOTUS nominee CNN's Toobin: Democrats are 'wimps' who won't 'have the guts' to add Supreme Court seats Republican senator says plans to confirm justice before election 'completely consistent with the precedent' MORE (D-N.Y.), who is expected to lead Senate Democrats in the next Congress.

Democrats note that the White House has chosen its words carefully when discussing how it will handle the bill.

“I don't take the administration's stated objections as a presidential veto threat,” said Democratic lobbyist Jack Quinn, who is working with 9/11 victims families backing the bill. “There is thus ample room here for the president to sign JASTA or let it become law without his signature.”

For months, the White House has lobbied against the measure and strongly hinted that Obama will nix it. But officials have stopped short of issuing a full veto threat.

“We have serious concerns with the bill as written,” a White House official said Wednesday,

“We believe there needs to be more careful consideration of the potential unintended consequences of its enactment before the House considers the legislation,” the official said. “We would welcome opportunities to further engage with the Congress on that discussion.”

Previous revisions to the Senate bill in May did not assuage the administration’s concerns, and it’s not clear how the legislation could be changed to win over Obama.

White House press secretary Josh Earnest has repeatedly said it would be “difficult to imagine a scenario in which the president would sign the bill,” but has not said it’s impossible.

Obama has rarely been more alone than he is now in a policy debate.

Vetoing the legislation would underline a disagreement with congressional Democrats and Clinton, who Obama hopes will succeed him in the White House.

“It would [surprise] me if she and the president found themselves at odds on this important anti-terrorism legislation,” said Quinn.

The White House hoped to have Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE (R-Wis.) in its corner after he expressed reservations over the measure this spring. But pressure from bill supporters has mounted on Ryan and other GOP leaders to move forward.

Under Obama, U.S. foreign policy has pivoted away from Saudi Arabia, straining ties with the kingdom. Saudi Arabia fiercely opposed the U.S. nuclear deal with Iran.

The Saudis have warned of further strain on its relationship with the U.S. if the 9/11 legislation becomes law.

Officials have reportedly threatened to sell off hundreds of billions of dollars in American assets in order to protect them from being frozen by court rulings, although economists doubt they would follow through.

Victims families have long sought to hold Saudi Arabia accountable for the attacks. Fifteen of the 19 hijackers were Saudi citizens, and there have long been rumors about ties between al-Qaeda and the government in Riyadh.

But bill opponents point out that 28 declassified pages of a congressional report released over the summer did not contain a smoking gun proving senior Saudi officials had a hand in the 9/11 attacks.

Former United Nations Ambassador John Bolton and ex-Attorney General Michael Mukasey, both of whom served under President George W. Bush, this week warned that the legislation “is far more likely to harm the United States than bring justice against any sponsor of terrorism.”

“There is already a law that permits U.S. citizens to sue any country our government has designated a state sponsor of terrorism, such as Iran,” they wrote in The Wall Street Journal. “JASTA, however, does not require a prior U.S. government designation, bypassing a critical safeguard to allow plaintiffs to get at the Saudis—and also setting a precedent for suits against other countries.”

If Obama vetoed the bill, he’d be going to bat for the Saudi regime — while taking on 9/11 victims’ families and large majorities in both chambers.

“I don’t think they’ll veto it — but they may veto it on the idea that they could say to the Saudis, ‘We’re in good faith and Congress is crazy,’ and do that as a basis to keep their relations with the Saudis,” said Rep. Steve Cohen (D-Tenn.), another original co-sponsor.

Some are asking whether Obama wants to expend political capital it desperately needs during the lame-duck session.

The president is hoping lawmakers will pass the Trans-Pacific Partnership trade agreement, a criminal-justice reform measure and confirm Supreme Court nominee Merrick Garland.

“They don’t want to pick a fight with the Democrats now,” Nadler said of the White House.

- This story was updated at 1:25 p.m.