Defense Secretary Contender James Mattis Faces Legal Obstacle to Getting the Post

— -- The saying is “Once a Marine, always a Marine,” and if President-elect Donald Trump picks retired Gen. James Mattis for secretary of defense, he is still too much a Marine in the eyes of the law.

Mattis retired in 2013, leaving him four years short of the requisite seven years after active duty before commissioned officers may serve as secretary of defense.

Experts say the reason for the mandatory break between active service and heading the Defense Department is to ensure that any incoming secretary has had time to adjust to being a civilian leader rather than a military officer.

“That’s an important principle in democratic politics just because sometimes the military itself is not the best judge of American foreign policy,” said David E. Lewis, a professor of political science at Vanderbilt University.

Trump met with Mattis on Saturday at the president-elect's estate in New Jersey and on Sunday tweeted that he was considering the retired Marine Corps general for the top defense post.

Now that Mattis has emerged as a contender, questions have arisen about whether he could legally be confirmed for the post.

If precedent holds, Congress would have to pass a law to exempt him from the requirement.

That’s what Congress did in 1950, when President Harry Truman nominated Gen. George C. Marshall for defense secretary. At the time, officers had to be out of active duty for at least 10 years before heading the Department of Defense.

The House and Senate passed the George C. Marshall Exemption Act, which made Marshall exempt from the waiting period.

Before the bill passed, however, Congress debated the wisdom of appointing a military officer as defense secretary.

According to a Congressional Quarterly Almanac entry from 1950, Rep. Frederick Coudert Jr. of New York argued, referring to Marshall, “No one man in 150 million Americans is indispensable, certainly not a 70-year-old man with one kidney who has given the best of his life” before retirement.

But others argued in favor of Marshall, citing his extensive credentials. Sen. Harry Byrd of Virginia asserted, “Besieged as we are by perils on every side, I feel it is our obligation, as representatives of the people, to place in this position of authority a man who, above all others, is best capable to perform the duties of secretary of defense.”

After passing the bill, the Senate had to vote to confirm Marshall, who had served as secretary of state. It took Congress five days for the two actions.

A Mattis nomination could spark an equally spirited debate. But Lewis said some members of Congress might prefer that Trump’s defense secretary have extensive military experience.

“It’s not as if Dwight Eisenhower is sitting in the presidency,” Lewis said, referring to the president, who had been a general.

ABC’s Ben Siegel and Luis Martinez contributed reporting.