President Barack Obama on Friday vetoed controversial legislation aimed at helping the families of the victims of the September 11th attacks sue Saudi Arabia — a move that sets up an emotionally-charged, election year showdown between an outgoing commander-in-chief and members of his party who supported the bill.

The measure, which was unanimously passed by both the House and Senate, enables the families of victims of the September 11th attacks to sue Saudi Arabia if that country is found legally liable for helping support the deadliest terrorist acts on U.S. soil. Fifteen of the 19 terrorists were Saudi and that nation's leaders have previously opposed the legislation and denied involvement.

The White House is strongly opposed to the legislation, known as the "Justice Against Sponsors of Terrorism Act", out of concerns it will open the floodgates and leave the U.S. vulnerable to similar suits. A bipartisan group of lawmakers backed the 9/11 families bill.

The president, in a statement explaining the veto on Friday, said he has "deep sympathy for the families of the victims of the terrorist attacks of September 11, 2001 (9/11), who have suffered grievously. I also have a deep appreciation of these families' desire to pursue justice and am strongly committed to assisting them in their efforts."