As always, national security is not a concern of this president….

President Obama on Thursday took the rare step of vetoing a major defense policy bill, upping the stakes in a faceoff with Republicans over government spending.

Obama used his veto pen on the National Defense Authorization Act during a photo-op in Oval Office.

“I’m going to be vetoing this authorization bill. I’m going to be sending it back to Congress, and my message to them is very simple: Let’s do this right,” Obama said.

It’s highly unusual for a president to veto the defense legislation, which typically becomes law with bipartisan support. The move amounts to a public rebuke of congressional Republicans, who warned that vetoing the $612 billion measure would put the nation’s security at risk.

The veto was Obama’s third this year and the fifth of his presidency. The Defense authorization bill has been vetoed four times in the last half-decade.

Obama argues the bill irresponsibly skirts spending caps adopted in 2011 by putting $38 billion into a war fund not subject to the limits, a move he called a “gimmick.” He has called on Congress to increase both defense and nondefense spending.

“Let’s have a budget that properly funds our national security as well as economic security, let’s make sure that we’re able in a constructive way to reform our military spending to make it sustainable over the long term,” Obama said.

The president also objects to language in the bill that requires the Guantanamo Bay, Cuba, military prison to remain open.

Republican leaders expressed outrage with Obama’s decision to veto the bill, pointing out that it puts a scheduled pay raise for troops, among other policy changes, at risk.

“By placing domestic politics ahead of our troops, President Obama has put America’s national security at risk,” Speaker John Boehner (R-Ohio) said in a statement.

“This indefensible veto blocks pay and vital tools for our troops while Iranian terrorists prepare to gain billions under the president’s nuclear deal.”

The move forces Congress to revisit the bill and send it back to the president. The military will continue to operate under last year’s defense policy if lawmakers cannot reach an agreement.

Republicans have pledged to attempt to override Obama’s veto, but it’s unlikely they have the votes to do so.

The Senate voted 70-27 to pass the bill, and overriding the veto would require 67 votes. But Democratic leaders have said some members would switch their vote to avoid defying the president.

The House vote count, 270-156, would not be enough to override a veto, which would take 290 votes.

Asked how confident the White House is Obama’s veto will be sustained, White House spokesman Eric Schultz replied: “very.”