Before retired Marine Gen. James Mattis can get a confirmation hearing to become the nation’s next secretary of defense, he’ll have to get past a decades-old law meant to ensure a cornerstone of American democracy — civilian control of the military.

The reasons behind putting a civilian in charge of the Department of Defense are numerous. The secretary of defense has immense responsibilities as the only person aside from the president who can authorize military action and is an influential voice in the decision to launch a nuclear strike.

The National Security Act of 1947 states that a secretary of defense will be appointed “from civilian life” by the president. The law calls for a grace period of ten years before an active duty officer can hold the post, though Congress knocked down the waiting period to seven years in 2008.

“The provision is a law because of America’s nervousness of giving the military too much power,” said Charles Stevenson, a professor at Johns Hopkins School of Advanced International Studies.

“There is a concern that someone who has been a general all their adult lives doesn’t really understand civilian life,” Stevens added. “The secretary of defense has to deal with domestic businesses, has to recruit people from the civilian job sector. If he is just used to commanding he might not be used to commanding civilian society.”

Releated: Trump Picks Tough Talking Gen. James 'Mad Dog' Mattis as Secretary of Defense

Mattis would be just the second retired general to lead the military. Army Gen. George C. Marshall, a five-star general, served as President Harry Truman’s secretary of defense from 1950 to 1951 as he oversaw the Marshall Plan aimed at rebuilding Europe after World War II.

But the waiver granted to Marshall came with a stern warning that the exception should not become the rule.

"It is hereby expressed as the intent of the Congress that the authority granted by this Act is not to be construed as approval by the Congress of continuing appointments of military men to the office of Secretary of Defense in the future," a report accompanying the 1950 statute stated. "It is hereby expressed as the sense of the Congress that after General Marshall leaves the office of secretary of defense, no additional appointments of military men to that office shall be approved."

Pleased Pres-elect has selected Gen Jim Mattis for #SecDef - one of the finest military officers of his generation https://t.co/3tOltWswgC — John McCain (@SenJohnMcCain) December 2, 2016

Similarly, Congressional aides say an exception allowing Mattis’ appointment would also make clear the provision does not approve the future appointment of military leaders to head the Pentagon.

Both Democrats and Republicans have largely reacted favorably to the Mattis selection, commending the four-star general for his experience and “warrior monk” reputation as a student of military strategy. But approving the necessary waiver has been met with skepticism from Democrats uncomfortable with upending a more than 60-year-old precedent. Further complicating the process is that the exception could be filibustered in the Senate and require 60 votes for passage. Though it may not derail the Mattis nomination, it could delay the process much more than if Trump chose a non-military nominee.

“While I deeply respect General Mattis’s service, I will oppose a waiver. Civilian control of our military is a fundamental principle of American democracy,” Sen. Kirsten Gillibrand, D-N.Y., said in a statement.

Sen. Ben Cardin, D-Md., called civilian leadership of the military “part of the fabric of America.”

“We know problems around the world where the military is too close to the governments of their countries. We’ve seen the consequence of that play out in very adverse ways,” Cardin said on MSNBC.

The waiver will be the only chance members of the House have to weigh in on one of Donald Trump's nominees. The Senate is responsible for approving Trump’s Cabinet, but both chambers will have a chance to cast ballots on the provision to dismiss the seven-year wait.

“While this [waiver] is something Congress should seriously consider, and I believe he would make an excellent Secretary of Defense, we must also bear in mind the precedent we would be setting and the impact it would have on the principle of civilian leadership of our nation's military,” Rep. Adam Schiff, D-Cali., said in a statement.