Updated

The battle in Congress over Donald Trump’s Cabinet selections already has begun – with the president-elect’s inauguration still weeks away.

The first skirmish broke out Tuesday over a Republican bid to help smooth the way for retired Marine Gen. James Mattis to be confirmed as Trump’s Defense secretary. Trump says he intends to nominate Mattis for the top Pentagon job, but this would require a waiver from Congress, since the Defense secretary by law is supposed to be a civilian far removed from military service.

Some Republicans are now trying to include language in a critical stopgap spending bill that would help facilitate that waiver. The broader spending bill would keep federal agencies functioning into next spring, with current funding set to expire at midnight Friday.

Some Democrats, however, see the Mattis provision as a poison pill and are fighting it.

“The American people are entitled to regular order and thoughtful scrutiny of nominees and any potential waivers,” House Democratic Leader Nancy Pelosi, D-Calif., said in a statement Tuesday. “Brushing aside the law that enshrines civilian control of the military – without discussion, in a massive must-pass funding bill – would set a terrible precedent. It is troubling that Republicans are working so hard to shield President-elect Trump’s choice for Secretary of Defense from the scrutiny and debate of Congress and the American people.”

The Republican measure being discussed would limit debate to 10 hours, but would still require 60 votes to approve the waiver for Mattis.

The Senate would still have to hold a separate vote to confirm Mattis as Defense secretary.

The reason Republicans and the incoming Trump administration would need a waiver traces back to the National Security Act of 1947. The law states “that a person who has within ten years been on active duty as a commissioned officer in a Regular component of the armed services shall not be eligible for appointment as Secretary of Defense.”

In 2008, Congress dropped that period from 10 to seven years. Mattis, who most recently served as head of U.S. Central Command, retired from the Marine Corps in 2013.

Trump announced last Thursday that he plans to nominate Mattis; the two were expected to appear together at a rally Tuesday night in North Carolina.

Though the waiver requirement threatens to cause delays, Mattis could still enjoy bipartisan support in the Senate. The pick won praise late last week. Sen. Lindsey Graham, R-S.C., one of Trump’s biggest GOP critics during the campaign, said Mattis would be “an outstanding choice.”

Meanwhile, the stopgap spending measure is considered the only must-do bill before Congress adjourns. As such, it's likely to carry several add-ons, including flood relief, money for overseas military operations and help for Flint, Michigan, to fix its lead-tainted water system as well as potential language on Mattis.

One major dispute centered on protecting health care and pension benefits for about 120,000 retired coal miners.

The measure had divided coal-state Republicans. Several supported the bill, but GOP leaders -- including Senate Majority Leader Mitch McConnell of Kentucky -- were wary of bailing out unionized workers.

McConnell said Tuesday that the temporary health care help for miners would be part of the spending bill, though Sen. Joe Manchin, D-W.Va., suggested the issue remained unresolved and continued to push for a permanent fix.

Manchin vowed to block any Senate effort to move quickly on unrelated legislation until the miners' fight was settled.

Fox News’ Chad Pergram and The Associated Press contributed to this report.