President Barack Obama on Friday vetoed a bill that would allow families of the victims of terrorist attacks in the United States to sue foreign governments believed to be linked to the strikes, setting up a difficult election-year decision for congressional Democrats.

Obama expressed “deep sympathy” for those who lost loved ones on Sept. 11, 2001, writing in a statement accompanying the veto that he has “deep appreciation of these families’ desire to pursue justice and [is] strongly committed to assisting them in their efforts.”

GOP presidential nominee Donald Trump called the veto “shameful,” and said he would have signed it into law. Earlier in the day, Democratic nominee Hillary Clinton broke with Obama and said she would have signed it.

Obama cited concerns that the legislation, which passed both the House and Senate with overwhelming bipartisan support, could prompt other nations to pass look-alike laws, leading to more lawsuits and inconsistent standards for what constitutes state support for terrorist attacks. Proponents, however, call it “narrowly” crafted to guard against such things.

He wrote that he could not sign the measure into law because it “would neither protect Americans from terrorist attacks nor improve the effectiveness of our response to such attacks.” Instead, Obama concluded, as crafted it would allow cases to be brought to U.S. courts against other countries based solely on allegations, not longstanding methods of determining state sponsors of terror.

What’s more, the president argued the bill could open the door for other governments to allow cases in their court systems against “U.S. officials — including our men and women in uniform — for allegedly causing injuries overseas via U.S. support to third parties.” Obama also said if it became law, the measure “threatens to create complications in our relationships with even our closest partners.”

Aides say Obama fully understands why convincing Democrats to block an override of his veto will likely be tough in an election year. “The president’s not blind to the politics of the situation,” White House Press Secretary Josh Earnest said Friday.

“The president understands that the talking points that are being prepared for the proponents of this bill have … political upside,” Earnest said. “But if we’re focused on the substantive long-term impact on our nation’s national security … that’s what’s driving the president’s decision to veto this bill. Not because it’s politically convenient. It’s not.”

The Senate’s No. 3 Democrat, Charles E. Schumer of New York, a co-sponsor of the measure, called the veto a “a disappointing decision,” predicting that it will be “swiftly and soundly overturned in Congress.”

“If the Saudis did nothing wrong, they should not fear this legislation,” Schumer said in a statement. “If they were culpable in 9/11, they should be held accountable. The families of the victims of 9/11 deserve their day in court, and justice for those families shouldn’t be thrown overboard because of diplomatic concerns.”

Senate GOP Whip John Cornyn of Texas, a co-author of the measure, criticized Obama for a “refusal to listen to the families of the victims taken from us on September 11, who should have the chance to hold those behind the deadliest terrorist attack in American history accountable.”

House Armed Services Committee Chairman Mac Thornberry backed Obama in opposing the bill. The Texas Republican circulated a dear colleague letter Friday saying he is concerned about the risk the bill poses to U.S. military, intelligence and diplomatic personnel.

Obama used up every day he had under law from the time the House unanimously passed the bill, formally dubbed the “Justice Against Sponsors of Terrorism Act,” to reject it. The move now sets up a decision for House and Senate leaders on when they will hold override votes.

“Now that we have received the veto message from the president, the Senate will consider [an override vote] as soon as practicable in this work period,” said David Popp, a spokesman for Senate Majority Leader Mitch McConnell in a statement on Friday.

McConnell said earlier this week that he believes there are ample votes in his chamber to override the veto.

On the House side, Speaker Paul D. Ryan of Wisconsin told reporters this week that there are enough votes in the House to pass a veto override.

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But the speaker appeared more nuanced in his stance than McConnell, saying: “I worry about legal matters.”

“I worry about trial lawyers trying to get rich off of this and I worry about the precedence,” Ryan said. “At the same time, these victims need to have their day in court.” Asked if the bill would get to the House floor this month, Ryan said the timing of such a vote depends on when the bill is taken up in the Senate.

Schumer told reporters earlier this week, “I think it will pass.”

“Look, the sooner, the better in my point of view,” he added, referring to the timing of the override votes.

Some have called the bill an attempt to allow the families of 9/11 victims to sue Saudi Arabia for its alleged ties to the Sept. 11 attacks.

The Obama administration contends that many lawmakers share its concerns on the measure.

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Earnest said Tuesday that members will soon have to decide whether the “votes they cast in public reflect the views they’ve expressed in private.” He cited conversations White House officials have had for months with members about the legislation.

The coming override votes will pit national security and foreign relations against domestic politics. That’s because, should both chambers vote before leaving for an October break to campaign, those facing re-election will likely see no choice but to vote to erase Obama’s veto pen stroke.

“My message to the [Republican] caucus is going to be: Unless there are 34 people willing to fall on their swords over this, [it’s] probably not worth falling on your sword over,” Tennessee GOP Sen. Bob Corker said.

It takes two-thirds of the Senate to support an override, and a source involved in the effort to persuade enough senators to vote to sustain Obama’s veto said the White House’s strategy is constantly changing.

A bit more about the White House’s lobbying strategy came into view Thursday, when the heads of two major U.S. companies, Dow Chemical and General Electric, wrote congressional leaders, warning that the bill could spawn negative consequences and damage relations with countries such as Saudi Arabia. 

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Earnest acknowledged this week that White House officials face an uphill battle and difficult optics in front of an electorate that surely would side with families of 9/11 victims, who support the legislation.

Notably, however, even House Democratic leaders, who typically stick with Obama, sound ready to reject his veto despite sharing some of his concerns.

House Minority Leader Nancy Pelosi said on Thursday that there was no political significance to the optics of members voting on the JASTA bill after the 15th anniversary of 9/11 and avoided questions on how she would vote to override the veto.

“This is difficult,” Pelosi said. “I think it’s going to happen.”

Pelosi joined other members in saying that she hasn’t heard from the White House on an expected veto by the president, but that the administration knows what lies ahead.

“The White House has not asked me to do anything on this,” Pelosi said.

Rep. Xavier Becerra, a California Democrat, on Wednesday expressed support for the bill but echoed Obama’s concern that it could also open Americans up to litigation in foreign courts.

“I would rather be in our courts than the courts of a lot of other countries,” Becerra said. “Many of us are prepared to vote for it, understanding that the sovereign immunity principle, which could impact us in the future could have some results that we don’t like.”

Rema Rahman, Rachel Oswald, Bridget Bowman and Lindsey McPherson contributed to this report.Contact Bennett at johnbennett@cqrollcall.com. Follow him on Twitter @BennettJohnT.