WASHINGTON — Congress on Wednesday voted overwhelmingly to override a veto by President Obama for the first time, passing into law a bill that would allow the families of those killed in the Sept. 11, 2001, terrorist attacks to sue Saudi Arabia for any role in the plot.
Democrats in large numbers joined with Republicans to deliver a remarkable rebuke to the president. The 97-to-1 vote in the Senate and the 348-to-77 vote in the House displayed the enduring power of the Sept. 11 families in Washington and the diminishing influence here of the Saudi government.
The new law, enacted over the fierce objections of the White House, immediately alters the legal landscape. American courts could seize Saudi assets to pay for any judgment obtained by the Sept. 11 families, while Saudi officials have warned they might need to sell off hundreds of billions of dollars in holdings in the United States to avoid such an outcome.
The override comes at an already freighted moment in America’s relations with the kingdom. The Saudi government has vigorously denied that it had any part in the Sept. 11 attacks, and the commission investigating the plot found “no evidence that the Saudi government as an institution or senior Saudi officials individually funded” Al Qaeda, the terror group that carried out the attacks. But the commission left open the possibility that some Saudi officials may have played roles.
Mr. Obama angrily denounced the outcome, saying lawmakers had been swayed to cast a political vote for legislation that set a “dangerous precedent” with implications they did not understand and never debated.
“I think it was a mistake, and I understand why it happened,” Mr. Obama said at a CNN town hall-style meeting with military personnel in Fort Lee, Va. “It’s an example of why sometimes, you have to do what’s hard, and frankly, I wish Congress here had done what’s hard. I didn’t expect it, because if you’re perceived as voting against 9/11 families right before an election, not surprisingly, that’s a hard vote for people to take. But it would have been the right thing to do.”
There were swift complications. Within hours of their vote, nearly 30 senators signed a letter expressing some reservations about the potential consequences of the law, including the prospect that the United States could face lawsuits in foreign courts “as a result of important military or intelligence activities.”
The White House and some lawmakers were already plotting how they could weaken the law in the near future, although there was general pessimism on Wednesday that Congress would agree to any changes. “You got to find consensus,” said Senator Lindsey Graham, Republican of South Carolina, after the vote. “Then you need a vehicle.”
It is unclear whether the Saudis will make good on warnings that the kingdom could unload hundreds of billions of dollars worth of assets inside the United States, and some economists have said that such a sell-off would do far more damage to Saudi Arabia’s economy than America’s.
But legal experts say there is cause for concern in Riyadh.
The law allows families of the Sept. 11 victims to alter lawsuits already underway — or file new suits — to directly sue Saudi Arabia and to demand documents and other evidence. It amends a 1976 law that grants foreign countries broad immunity from American lawsuits. Now nations can be sued in federal court if they are found to have played any role in terrorist attacks that killed Americans on United States soil.
“From there, the ball goes squarely into the Obama administration’s court,” said Stephen I. Vladeck, a professor at the University of Texas School of Law.
As Mr. Vladeck noted, a little-discussed provision of the bill allows the attorney general to intervene in the lawsuits and get a judge to stay any settlement as long as there are continuing discussions with the Saudis about a possible resolution.
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The provision was added earlier this year to soften the legislation — preserving the executive branch’s purview over foreign policy while still giving family members a path to sue.
But the prospects of such discussions ever beginning are uncertain. The Saudi government has long denied any role in the Sept. 11 plot, and any negotiation with the United States could be viewed as acknowledging culpability.
At the same time, lawyers for the families will no doubt push for judges to carefully scrutinize any attempt by the attorney general to delay court proceedings.
“The families would of course expect that in the event the provision is invoked, that the courts exercise their inherent authority to assure good faith negotiations are in fact taking place and that the courts not simply rubber stamp executive branch requests for delay in resolution of their claims,” said Allan Gerson, who is part of a team representing many of the Sept. 11 families.
Mr. Gerson filed a lawsuit against Libya on behalf of families of the victims of Pan Am Flight 103, which was brought down by a bomb as it flew over Lockerbie, Scotland, in 1988.
In recent days, Mr. Obama, Defense Secretary Ashton B. Carter and Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, all wrote letters to Congress warning of the dangers of overriding the veto.
John O. Brennan, the C.I.A. director, released his own statement saying, “Any legislation that affects sovereign immunity should take into account the associated risks to our national security.”
Yet even most of Mr. Obama’s strongest allies on Capitol Hill turned against him.
“This is a decision I do not take lightly,” said Senator Chuck Schumer, Democrat of New York, one of the legislation’s authors. “This bill is near and dear to my heart as a New Yorker, because it would allow the victims of 9/11 to pursue some small measure of justice, finally giving them a legal avenue to pursue foreign sponsors of the terrorist attack that took from them the lives of their loved ones.”
Only one senator, Harry Reid, Democrat of Nevada, sided with the president.
The bill succeeded not with significant congressional debate or intense pressure from voters, but rather through the sheer will of the victims’ families, who seized on the 15th anniversary of the attack and an election year to lean on members of Congress. That effort was aided by lawmakers’ waning patience with the kingdom in recent years.
The override also represents a White House miscalculation about Capitol Hill, where it was slow to pressure members and see the cracks in its firewall against the bill.
The Senate voted unanimously in May to pass the bill and send it to the House, but many lawmakers and White House officials believed the chamber would never take up the legislation.
Then this month, Speaker Paul D. Ryan, who had encountered families of the Sept. 11 victims at a fund-raiser on Long Island, decided to put the bill on a fast track. The House voted overwhelmingly in favor, sending it to Mr. Obama’s desk.
Senator Bob Corker, Republican of Tennessee, gave voice to the unusual ambivalence that many members of Congress have expressed since they passed the bill.
“I do want to say I don’t think the Senate nor House has functioned in an appropriate manner as it relates to a very important piece of legislation,” said Mr. Corker, the chairman of the Senate Foreign Relations Committee. “I have tremendous concerns about the sovereign immunity procedures that would be set in place by the countries as a result of this vote.” Still, it was a vote he cast.
Correction: September 28, 2016
An earlier version of this article incorrectly referred to how Pan Am Flight 103 was attacked in 1988. It was brought down by a bomb as it flew over Lockerbie, Scotland, not by being shot down.