Democrats may not support the waiver necessary for Donald Trump to appoint former Marine Gen. Jim Mattis as secretary of defense, a leading voice for national security issues in Congress said Tuesday, bringing into question whether the president-elect could get the support on Capitol Hill he needs to effectively change the law.

"Personally, I like him very much, I have great respect for him. I think he very well may make an excellent secretary of defense," Rep. Adam Schiff, ranking member of the Select Committee on Intelligence, said Tuesday morning. "What may tip the balance for me is if the president-elect populates even more positions within the administration with even more military figures, I'm going to have a lot of heartburn about this."

"If he starts making these other nominations, it's going to place the Mattis waiver in real jeopardy, at least as far as Democrats are concerned," the California Democrat said while speaking at a breakfast organized by the Christian Science Monitor.

Trump has already nominated former Defense Intelligence Agency chief Mike Flynn, a retired three-star Army general fired from that position in 2014, to serve as his national security adviser. He's currently weighing at least six other former flag officers for positions in his Cabinet, including disgraced retired Army Gen. and former CIA Director David Petraeus for the post of secretary of state, and current NSA head Navy Adm. Mike Rogers, who has reportedly fallen out of favor with some of President Barack Obama's top advisers .

U.S. law prohibits former military officials from becoming secretary of defense within seven years of doffing their uniform. Congress has made an exception only once since the law was passed in 1947, approving a waiver in 1950 that allowed for George Marshall to become Pentagon chief. Lawmakers stipulated in the law their intent that the waiver be a one-time change and that "no additional appointments of military men to that office shall be approved."

It remains unclear precisely how Congress would approve a waiver for Mattis and whether that would happen before the new administration takes office Jan. 20.

The House and Senate would have to approve allowing Mattis before the Senate ultimately votes on his nomination at some point next year. The waiver, which is actually a change to the law, could originate in either the House or the Senate, as a standalone bill or as a rider on a larger bill, the sources say.

The language could be tacked on to the defense bill or be included in a continuing resolution were Congress to vote for one, though the rider on either would have to be added before the vote this Friday.

"Barring the president militarizing the rest of his Cabinet – and I think the most important position will be the secretary of state position – I would be inclined to make my decision based on Gen. Mattis' merits and a willingness to make another one-time exception," Schiff said.

Others very familiar with the process have called into question how strictly Congress needs to enforce the current law.

"Who the hell are we kidding? Seven years? Where the hell did that come from?" Leon Panetta said with his characteristic bluntness over the weekend at The Reagan Defense Forum in his native California.

The former Democratic congressman and head of the House Budget Committee, Office of Management and Budget, CIA and Pentagon emphasized that the purpose of the law is to enforce longstanding U.S. tradition that the government must be seen as run by civilians, not military leaders. Congressional hearings in the course of providing the waiver to Mattis could provide legislators with the kind of assurances they might need that he would understand his responsibilities as a civilian, not a combat commander, Panetta said.