Congress on Friday sent President Obama a bill that would allow families of the victims of the 9/11 attacks to sue Saudi Arabia over its alleged ties to terrorism, but advocates of the legislation worry it could be defeated by a presidential veto.

The House passed the legislation by voice vote, with leaders calling it a “moral imperative” to allow victims’ families to seek justice for the deaths of loved ones as the country marks the 15th anniversary of the attacks that brought down the World Trade Center towers and damaged the Pentagon.

But bill supporters are bracing for a veto fight with the White House, which argues the bill could harm the United States’ relationship with Saudi Arabia and establish a legal precedent that jeopardizes American officials overseas. Advocates for the legislation are also warily eyeing the congressional calendar over fears the administration may try to pocket-veto the legislation if lawmakers leave Washington soon to focus on the election.

Victims’ families who have long implored Congress to pass the bill are now pressuring lawmakers to stick around Washington to prevent a pocket veto, arguing they have enough support for a successful override vote.

House Speaker Paul Ryan (R-Wisc.) and Minority Leader Nancy Pelosi (D-Calif.) spoke at a September 11 remembrance ceremony at the U.S. Capitol on Sept. 9. (Reuters)

“This is more important than campaigning,” said Terry Strada, who lost her husband in the attacks and is national chair of the organization for victims’ families that is bringing a lawsuit against Saudi Arabia. “You can campaign after. You will never have a chance to pass [the bill] again. This is the priority.”

Once the president officially receives the bill, he will have 10 days to veto the legislation or the bill automatically becomes law. But if Congress adjourns before the 10-day clock runs out, it could trigger a pocket veto — a constitutional quirk that allows a president to defeat a legislative proposal by holding on to it until Congress is out of session.

The White House is in a difficult spot. While administration officials have strongly suggested the president should veto the bill, it would be a politically unpopular move that could fuel an emotional backlash and an uncomfortable debate in the weeks before Election Day.

A pocket veto could help the White House avoid some of the political fallout but would also probably prove controversial.

White House National Security Council Spokesman Ned Price on Friday declined to comment on whether the president will veto the bill.

The bill would allow courts to waive claims to foreign sovereign immunity in cases involving terrorist attacks on U.S. soil. Saudi Arabia has lobbied hard against the legislation, even threatening to start selling off U.S. assets if the measure passes. The White House has threatened a veto on the rationale that it could put U.S. diplomatic officials in a bad position if countries respond by similarly ignoring the established practice of granting immunity to foreign government representatives.

“This administration strongly continues to oppose this legislation, and, you know, we’ll obviously begin conversations with the House about it,” White House press secretary Josh Earnest said in May when the Senate passed the bill.

Advocates for the legislation dispute the validity of the White House’s arguments, pointing out that countries that have done nothing wrong and don’t support terrorists have nothing to worry about.

Since the Senate passed the legislation in May by unanimous consent, the government has released a previously classified set of pages from a 2002 congressional inquiry into the 9/11 attacks that dealt with suspected connections between Saudi Arabia and the terrorists involved. Those pages did not significantly add to the information that had already been made public through other documents and reports.

But victims’ families want to use the courts to further explore any ties between the 9/11 attackers and the Saudi government or Saudi officials.

While the House is scheduled to stay in session through September, the Senate could disband as early as the end of next week so that lawmakers can focus on their campaigns through Election Day, according to Sen. Roger Wicker (R-Miss.), who chairs the Senate Republicans’ campaign operation.

A Senate GOP leadership aide said Friday that the Senate planned to hold pro forma sessions while Congress is out of town to prevent the White House from making a pocket veto or staging any other surprise moves.

But there is some dispute over whether pro forma sessions are enough to prevent the administration from attempting a pocket veto.

In late 2007, President George W. Bush claimed the right to pocket-veto a defense spending bill over the protests of congressional leaders. At the time, the House had adjourned for a holiday break and the Senate was holding pro forma sessions every few days — a practice that has become standard to prevent the White House from making recess appointments.

Because the 9/11 bill started in the Senate, leaders there are confident the pro forma sessions will prevent anyone from arguing that Congress has legally adjourned, as defined by the Constitution.

Victims’ families say they are pressuring the White House to back off its veto threat while also asking lawmakers to stay in town in case an override vote is needed.

“I don’t believe they’re going to let us down,” Strada said. “I don’t think they would have done all this work just to let it fall apart at the end.”

Mike DeBonis contributed to this report.