President Trump followed through Wednesday on his threat to veto a massive annual defense policy bill, setting up what could be the first, and potentially only, veto override of his presidency.

Congress passed the fiscal 2021 National Defense Authorization Act (NDAA) earlier this month with more than the two-thirds majority needed to override a veto, though it is unclear how many Republicans will buck the president in a planned veto override vote.

“My Administration recognizes the importance of the Act to our national security,” Trump said in a message notifying Congress of the veto. “Unfortunately, the Act fails to include critical national security measures, includes provisions that fail to respect our veterans and our military's history, and contradicts efforts by my Administration to put America first in our national security and foreign policy actions.”

Trump previously objected to the $740 billion policy legislation because it did not include a provision repealing Section 230 of the Communications Decency Act, a 1996 law that provides a legal shield to tech companies like Twitter and Facebook.

Trump had also threatened to veto the legislation because it included a provision requiring military bases named for Confederate to be renamed within three years, and he recently complained that he viewed the bill as weak on China, despite numerous provisions aimed specifically at Beijing.

Trump said in Wednesday's message that the mandated changes to Confederate-named bases amounted to a politically motivated attempt “to wash away history and to dishonor the immense progress our country has fought for in realizing our founding principles.”

He also called the bill a “gift" to Russia and China.

Trump, who had 10 days excluding Sundays to act after Congress sent him the bill, waited until the last day to veto it.

Trump became fixated on repealing Section 230 after Twitter started adding labels to his posts that made unsubstantiated allegations of widespread voter fraud, arguing in his veto message that the statute poses a “very dangerous national security risk.”

But lawmakers in both parties resisted his demands to address the issue in the defense bill, saying it was not the place to tackle a thorny, unrelated tech policy.

Trump issued the veto message shortly before he was scheduled to depart Washington for his Mar-a-Lago club in Palm Beach, Fla., where he is scheduled to spend the Christmas holiday. The veto message came amid percolating tensions between the president and Republican leaders after he eviscerated a newly passed bipartisan government funding and COVID-19 relief measure in a video posted to Twitter late Tuesday.

“The NDAA has become law every year for 59 years straight because it’s absolutely vital to our national security and our troops. This year must not be an exception,” Senate Armed Services Committee Chairman James Inhofe (R-Okla.) said in a statement Wednesday after Trump’s veto. “I hope all of my colleagues in Congress will join me in making sure our troops have the resources and equipment they need to defend this nation.”

In order to override Trump's veto, both the Senate and House would need two-thirds of their members to vote in favor of overriding it. The bill already passed the Senate in an 84-13 vote and the House in a 355-78 vote.

Congress must override the veto before noon on Jan. 3, when the 117th Congress will be sworn in. If Congress fails to override the veto by then, lawmakers would need to start from scratch on the bill.

The House plans to hold its veto override vote Monday in a rare post-Christmas session.

If the House successfully overrides Trump, the Senate is planning to come back the following day. But it could still be days after that before the upper chamber holds a final override vote if senators who support Trump’s veto drag out procedural hurdles. Senators have suggested the override vote could happen the morning of Jan. 3, just before the new Congress is sworn in.

Sen. Rand Paul (R-Ky.), who briefly held up passage of the NDAA earlier this month, indicated Monday he could similarly delay an override vote.

“I very much am opposed to the Afghan war, and I’ve told them I’ll come back to try to prevent them from easily overriding the president’s veto,” Paul told reporters.

Sen. Lindsey Graham (R-S.C.), one of the president’s closest allies on Capitol Hill who did not vote on final passage of the NDAA, tweeted that he would not vote to support the veto override unless Congress made an effort to “wind down” Section 230.

House Armed Services Committee Chairman Adam Smith (D-Wash.) projected confidence Wednesday that Congress will ultimately override Trump.

“By choosing to veto the NDAA, President Trump has made it clear that does not care about the needs of our military personnel and their families,” Smith said in a statement. “The FY21 NDAA passed with overwhelming, veto-proof support in both the House and Senate, and I remain confident that Congress will override this harmful veto. While the president may not care about our service members and their families, Congress still places an immense value on their service and sacrifice.” 

The bill, which has become law for 59 years in a row, is considered must-pass because it authorizes a host of special pay and bonuses for troops, military construction projects, training programs and other vital operations.

Trump’s opposition to the legislation caused tensions between the White House and Senate Republicans in the waning weeks of his presidency.

"My intention was and is to ensure the Senate continues fulfilling our obligation to the men and women of our armed forces. I hope the president will not veto this bill," Senate Majority Leader Mitch McConnell (R-Ky.) said from the Senate floor earlier in the week.

Republicans indicated earlier that there had been efforts to persuade Trump to back down from his veto threat, and they were hoping a strong enough vote would convince him to sign the bill.

They ultimately were unsuccessful.

Trump’s decision to go through with the veto is likely to further exacerbate tensions between the White House and Capitol Hill at a time when Trump’s refusal to concede to President-elect Joe Biden and effort to overturn the election results has divided Republicans.

Trump has also recently put Republicans in a difficult position by objecting to the $2.3 trillion coronavirus relief and government funding package passed by both chambers earlier this week after months of stop-and-start negotiations.

It is unclear whether Trump will veto that legislation as well once it reaches the White House.

Some Republicans who voted to pass the NDAA, such as House Minority Leader Kevin McCarthy (R-Calif.), have said they would not support a veto override of the defense bill.

Some Democrats who voted against the NDAA have said they would switch their votes to overcome Trump’s veto.

Updated at 4:07 p.m.