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."

However, the measure would "invite consequential decisions to be made based upon incomplete information and risk having different courts reaching different conclusions about the culpability of individual foreign governments and their role in terrorist activities directed against the United States — which is neither an effective nor a coordinated way for us to respond to indications that a foreign government might have been behind a terrorist attack," the president continued.

9/11 Families & Survivors United for Justice Against Terrorism, a group representing the families of victims said in a statement Friday that they are "outraged and dismayed" at the president's veto and called his reasons "unconvincing and unsupportable".

Related: House OKs Bill to Let 9/11 Families Sue Saudi Arabia

The upcoming congressional face-off also puts an added degree of pressure on Senate Democrats who, though they supported the initial legislation, might be concerned about having a hand in the first veto override of Obama’s presidency.

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The president will have to convince 34 of them to change their positions and see things his way.

Democratic presidential nominee Hillary Clinton said she supports the measure.

"Clinton continues to support the efforts by Senator Schumer and his colleagues in Congress to secure the ability of 9/11 families and other victims of terror to hold accountable those responsible. She would sign this legislation if it came to her desk," Clinton spokesman Jesse Lehrich said in a statement.

Republican presidential nominee Donald Trump criticized the veto as "shameful" and said the families deserve their day in court.

"That President Obama would deny the parents, spouses and children of those we lost on that horrific day the chance to close this painful chapter in their lives is a disgrace," Trump said in a statement.

White House press secretary Josh Earnest earlier this week said the president and members of his national security team have had ongoing conversations with members of Congress about the legislation and expressed their concerns. The White House worries that the nation’s diplomats and even business leaders could be put at risk by such legislation.

Related: Could Obama Face Veto Override Showdown Over 9/11 Bill?

“As we've made this case, members of Congress in both parties have indicated that they are open to the concerns that we've expressed — in many cases, they share them,” Earnest told reporters on Tuesday. “And the real question for members of Congress will be whether or not they're prepared to cast a vote that is consistent with the views and feelings that they express in private.”

The administration says getting its point across about the measure is a “steep hill.”

House Speaker Paul Ryan on Wednesday said he does “think the votes are there for the override” and if the Senate takes up the override this month “then it is going to come over here and we will pass it”. And earlier this year, Sen. Chuck Schumer, D-New York, a co-sponsor of the bill, told reporters he felt confident the Senate could override a presidential veto.

Mindy Kleinberg, who lost her husband on 9/11 and protested earlier this week in Washington D.C. in effort to get Obama to sign the bill, told NBC News, "I'm greatly disappointed. I would have liked him to stand with us. To come to this point where he's the one man standing between the way of justice and making the nation is horrifying."

Kleinberg said, however, that she was optimistic Congress would override Obama's veto.

She added family members of 9/11 victims next week would be meeting with lawmakers to urge them to continue to back the legislation.

Attorney James Kreindler, co-chair of the plaintiffs' executive committee representing the 9/11 families and victims, said he was confident Congress would override Obama's veto, predicting "It's not going to even be close."

Kreindler said a new complaint would be filed against Saudi Arabia and his team would begin to put in discovery requests.

"The facts are already so overwhelming. Saudi Arabia doesn't want to see this continue in the media or court...We're going to prevail. We are going to win. Either the Saudis will come to the table or we'll go to court and win there," he said.

Before Friday, Obama had previously issued 11 vetoes during his presidency according to Senate records— none of which had been overturned.

In comparison, George W. Bush issued 12 vetoes during his tenure, four of which were overturned during his last two years in office as he saw his popularity plummet. Bill Clinton issued 36 vetoes with two overridden and George H.W. Bush issued 29 vetoes with just one overridden.