Obama faces real prospect of Republican and Democratic lawmakers joining forces to override his veto for first time in his presidency
President Barack Obama on Friday vetoed a bill allowing families of victims of the 9/11 attacks to sue Saudi Arabia, risking fierce public backlash and a rare congressional rebuke.
While expressing "deep sympathy" for the families, Obama said the law would be "detrimental to US national interests".
The White House tried and failed to have the legislation - which was unanimously passed by Congress - scrapped or substantially revised.
Terry Strada, whose husband Tom was killed in World Trade Center Tower One, told AFP the 9/11 "families are outraged and very disappointed" by Obama's decision.
She vowed that the group would now lobby "just as hard as we possibly can" to have Congress overturn the decision.
Obama now faces the very real prospect of Republican and Democratic lawmakers joining forces to override his veto for the first time in his presidency.
Such overrides are rare. Obama has issued 12 vetoes so far in his presidency, none of which have garnered the two-thirds opposition needed for an override.
New York Senator Chuck Schumer - a Democrat with close ties to Obama and who co-sponsored the bill - insisted that is about to change.
"This is a disappointing decision that will be swiftly and soundly overturned in Congress," he said.
"If the Saudis did nothing wrong, they should not fear this legislation. If they were culpable in 9/11, they should be held accountable."
Families of 9/11 victims have campaigned for the law - convinced that the Saudi government had a hand in the attacks that killed almost 3,000 people.
Fifteen of the 19 hijackers were Saudi citizens, but no link to the government has been proven. The Saudi government denies any links to the plotters.
Declassified documents showed US intelligence had multiple suspicions about links between the Saudi government and the attackers.
"While in the United States, some of the 9/11 hijackers were in contact with, and received support or assistance from, individuals who may be connected to the Saudi government," a finding read.
Behind the scenes, Riyadh has lobbied furiously for the bill to be scrapped.
A senior Saudi prince reportedly threatened to pull billions of dollars out of US assets if it becomes law, but Saudi officials now distance themselves from that claim.
Republican nominee Donald Trump has already tried to paint Obama and his would-be successor Hillary Clinton as weak on terrorism.
Clinton has preemptively voiced support for congressional efforts "to secure the ability of 9/11 families and other victims of terror to hold accountable those responsible," according to Jesse Lehrich, a campaign spokesman.
In a diplomatic protest note obtained by AFP, the European Union warned the rules would be "in conflict with fundamental principles of international law".
"State immunity is a central pillar of the international legal order," the "demarche" noted, adding that other countries could take "reciprocal action".
In a letter to lawmakers, also seen by AFP, former Secretary of Defence William Cohen, former CIA boss Michael Morell and Stephen Hadley, George W Bush's national security adviser, were among a group of high-profile security figures warning that the legislation would hurt US interests.
"Our national security interests, our capacity to fight terrorism and our leadership role in the world would be put in serious jeopardy," they said.