Arizona Republican Sen. John McCain said Tuesday he is concerned about the looming confirmation process for retired Marine Gen. James Mattis, who President-elect Trump says he will nominate to become the next secretary of defense.

McCain, the chairman of the Senate Armed Services Committee, told reporters in the Capitol he is "very worried" about Mattis' confirmation process. Mattis will have to receive a waiver allowing him to serve in a civilian position less than seven years after retiring from the military.

"I'm very worried about it," McCain said when asked about receiving a waiver for Mattis. "We have to work on it."

Mattis left active service in 2013 after concluding his time as commander of Central Command, forcing the need for a waiver. George C. Marshall, who received one in 1950 in order to also become secretary of defense, is the only person to have received one in the past.

"This is probably the most important post, obviously, of all the cabinet positions, so we should expedite this process just like how I expedited every other process when Obama proposed nominees for the secretary of defense," McCain said.

"I was active in helping move that process forward as quickly as possible. The Democrats should do the same for us," McCain continued. "And when they don't, and we have to resort to parliamentary action to force them to, then that's disgraceful because we're talking about defending our nation. And when you block a secretary of defense for some reason, you go back to our reluctance to confirm a nominee for the United States Supreme Court, there's no connection between the two, and it's disgraceful."

The Arizona senator was referring comments made by Sen. Sherrod Brown, D-Ohio, who openly questioned why Democrats would support Trump's cabinet selections after Republicans refused to bring Merrick Garland's nomination up for consideration after the death of longtime Justice Antonin Scalia.

Democratic opposition to a waiver started soon after Trump announced his intention to nominate the retired Marine to the post. Sen. Kirsten Gillibrand, D-N.Y., came out immediately against granting Mattis a waiver, citing the need to have a civilian heading the position.