WASHINGTON (AP) — Congress is poised to override President Barack Obama’s veto of a bill that would allow families of Sept. 11 victims to sue Saudi Arabia for the kingdom’s alleged backing of the terrorists who carried out the attacks that killed nearly 3,000 people.

The showdown is scheduled for Wednesday. Proponents of the legislation say they have enough votes for what would be a first: During his nearly two full terms in office, Obama has vetoed nine bills. None has been overridden.

While there is broad and bipartisan support for bucking the president, the bill’s opponents also are pushing hard to keep the measure from being enacted. They’re warning the US will become vulnerable to retaliatory litigation in foreign courts that could put American troops in legal jeopardy.

Here’s a look at the key issues surrounding the bill, the Justice Against Sponsors of Terrorism Act, as the veto override vote nears:

What would the legislation do?

The legislation, known as JASTA, gives victims’ families the right to sue in US courts for any role that elements of the Saudi government may have played in the 2001 attacks that killed thousands in New York, the Washington, DC, area and Pennsylvania. Under the terms of the bill, courts would be permitted to waive a claim of foreign sovereign immunity when an act of terrorism occurs inside US borders. Saudi Arabia has objected vehemently to the bill.

Why did Obama veto the bill?

In his veto message issued on Friday, Obama said the bill would disrupt longstanding international principles on sovereign immunity and could create complications with even the closest allies of the United States.

Foreign governments would be able to act “reciprocally” and allow their courts to exercise jurisdiction over the United States and its employees for allegedly causing injuries overseas through American support to third parties, according to Obama. As examples, Obama cited actions taken overseas by US-backed armed militias, the improper use of US military equipment, and abuses committed by US-trained police units.

The bill’s proponents have disputed Obama’s rationale as “unconvincing and unsupportable,” saying the measure is narrowly tailored and applies only to acts of terrorism that occur on US soil.

Senate Majority Leader Mitch McConnell said Monday that the Senate would vote Wednesday on the president’s veto.

What’s the concern for American troops and secrets?

Rep. Mac Thornberry, the Texas Republican who chairs the House Armed Services Committee, said that even if none of the potential lawsuits against the United States succeeded, “the risks of discovery or trial in foreign courts, including the questioning of government employees under oath, will disclose sensitive information and subject Americans to legal jeopardy of various kinds.”

Thornberry is opposed to the bill and is urging his colleagues not to override Obama’s veto.

But attorneys for the 9/11 families said US military personnel are not at risk of lawsuits. Should a foreign government enact a law that allows a claim against American service members, that nation would not be reciprocating but engaging in a “transparent and unjustifiable act of aggression” that the US should respond to, they said.

House Majority Leader Kevin McCarthy, R-California, said Monday he is inclined to vote to override and said it looks as though there are enough votes in the House to overturn the president’s decision. The House would act after the Senate votes.

Is there heightened tension with a key Middle East ally?

An override of Obama’s veto is stoking apprehension about undermining a longstanding yet strained relationship with Saudi Arabia, a critical US ally in the Middle East. Saudi Foreign Minister Adel bin Ahmed Al-Jubeir had previously warned lawmakers they were on a path to turning “the world for international law into the law of the jungle.”

Sen. Lindsey Graham, R-South Carolina, last week advised his congressional colleagues against alienating Saudi Arabia at a time when the US needs the kingdom’s support to defeat Islamic State militants. “If you want to lose Saudi Arabia as an ally, be careful what you wish for,” said Graham, a member of the Senate Armed Services Committee. “They’re the heart and soul of the Islamic world.”

How many votes are needed to override a veto?

A two-thirds majority of lawmakers present and voting is required in the House and Senate to override a veto.

Obama has vetoed the fewest bills — just nine — since President Warren G. Harding was in office more than 90 years ago, according to a web page maintained by the offices of the House clerk and historian. Harding served just two years as president before dying of a heart attack in 1923.

By comparison, President Bill Clinton vetoed 37 bills and George W. Bush vetoed a dozen. Lyndon Johnson is the last president to never have a veto overridden.