Section 903(a) of the National Defense Authorization act
requires a seven-year wait period between active duty and serving as Secretary of Defense -- a civilian role -- for former service members. Mattis would need a special waiver from Congress, since he retired from the military in 2013.
An aide to the House Armed Services Committee told CNN that they are conducting a review to examine how a waiver might be granted. The aide said no one from the President-elect's transition team had reached out to the committee, but committee staffers have started reviewing what would be required.
Mattis wouldn't be the first defense secretary to require a congressional waiver. President Harry Truman appointed Gen. George Marshall to the position in 1950, nearly five years after he retired from the Army. Congress granted the waiver (at the time, the interval between active duty and defense secretary service was 10 years).
As was the case with Marshall, a new law would have to be passed by both the House and Senate to enable the appointment of a military officer who had not been out of uniform for seven years.
However, the aide doesn't anticipate much resistance in Congress.
"Mattis is pretty popular among the members," the staffer said. In Marshall's case, he noted, the whole process took only "a couple of days."
As chairman of the Senate Armed Services Committee, Sen. John McCain would play a role in issuing the waiver for Mattis. On Monday, McCain released a statement says Mattis is "one of the finest military officers of his generation and an extraordinary leader."
"I hope he has an opportunity to serve America again," McCain said.
Why this statute exists
Former House Intelligence Committee Chairman Mike Rogers explained on "New Day" that this statute was put in place in 2008 because "you don't want a military attitude in the civilian position."
"When you get to that Defense Secretary role, it has to be a broader, strategic impact brought to any decision you make in any strategic event you make around the world -- including, by the way, acquisitions of weapons," Rogers said. "I think they just want a little distance there to make sure you don't just take a general and make them secretary. And that's why, originally, the law was passed to give that seven-year window."
Former Secretary of Defense Bill Cohen told CNN's Brianna Keilar, "There's nothing magical in the seven years. They want to have enough time to say: Are you separate enough from the military ethic and culture and part of the community as such to be the civilian boss?"
Rogers cautioned that getting that waiver from Congress could become a costly use of political capital.
"Just remember, you have to invest political capital to get your nominee a waiver, and then you have to get your nominee through the Senate. So that's two different processes. And that should be a factor when you are considering if you want this person to be your next Secretary of Defense." Still, Rogers praised Mattis as a "strategic thinker" with a "stellar reputation."
"Again, this is a very tough warrior who is a strategic thinker that thinks about the second and third order effects of every decision when you use your military," Rogers said. "That's why I think he's attractive to the Trump folks and why he would, if he could go through this process, would be a really good Secretary of Defense."
Maj. General James "Spider" Marks, who was on "New Day" with Rogers, said the retired general's confirmation was "worth the political capital."
"Nobody is more well prepared than Gen. Jim Mattis," Marks said. "He truly is a student of the profession and has demonstrated that throughout the course of his career without a blemish ... He is absolutely no nonsense, he's a very nuanced thinker, but he is very clear in how he approaches problem solving."
"Frankly," Marks added, "the military needs to shore up its relationship with the administration." He said the Trump administration needs more leadership, and Mattis would bring that to the table "immediately."