President Trump's new acting White House chief of staff Mick Mulvaney isn't giving up his old job as White House budget director, meaning he can be forced to testify before Congress when Democrats retake the House of Representatives next month.

White House chiefs of staff generally are off-limits to congressional subpoenas, but Mulvaney's decision to be both acting chief of staff and director of the Office of Management and Budget sets up potentially ugly fights over his testimony.

"I think this is a huge mistake for Mulvaney and for the president because it creates a real possibility for separation of powers brinksmanship," said American Oversight executive director Austin Evers.

"If he doesn't want to get into this complicated constitutional fight and potentially engage in obstruction, then he should resign and just focus on being chief of staff," said Evers, a former State Department senior counsel focused on transparency and oversight.

Traditionally, senior White House staff aren't subpoenaed by Congress in deference to the president's right to confidential advice. But officeholders confirmed by the Senate — such as the OMB director — are presumed to be available for questioning.

Louis Fisher, senior specialist in separation of powers at the Library of Congress from 1970 to 2006, said, "Mulvaney certainly occupies a number of positions, but I see no reason why he cannot be subpoenaed by Congress."

The OMB director became a Senate-confirmed position under President Richard Nixon, with lawmakers expressly seeking oversight power, said University of Baltimore law professor Charles Tiefer, solicitor and deputy general counsel of the House of Representatives from 1984 to 1995.

"He's top banana and a Democratic House will be active," Tiefer predicted.

Tiefer said it's customary for high-ranking officials to be considered too busy to testify before subcommittees but that a full committee's request could trigger a struggle. For example, he said, incoming House Financial Services Committee Chairwoman Maxine Waters, D-Calif., a strong critic of Trump's, could try to force an appearance from Mulvaney.

"When she fires off her cannons, it’s going to sound on the receiving end like a 21-gun salute. What it comes down to in that situation is scheduling," Tiefer said. "[Mulvaney] may say his schedule is all booked up for the next couple months … If she won't go along, then there could be an angry brawl over the scheduling book."

Tiefer said, "It’s not a bold prediction to say the words executive privilege are going to be spoken a lot from the lips of President Trump.”

Tom Fitton, president of the conservative transparency group Judicial Watch, said that "subpoenas are always a battle for high-level administration officials, especially senior White House staff that directly advise a president. Mulvaney's being chief of staff may mean that executive privilege will be invoked."

If Mulvaney refuses to testify, Democrats could vote to hold him in contempt. But Congress cannot do much on its own to enforce the finding. House Republicans voted in 2012 to hold then-Attorney General Eric Holder in contempt for failure to comply with a subpoena for documents, but the Justice Department, led by Holder, chose not to pursue the criminal referral.

Liz Hempowicz, director of public policy at the Project on Government Oversight, agreed that Mulvaney could be subpoenaed, at least about his OMB work, but said, "You don't just wave around a subpoena and get everything you want."

"I think the harder part with the subpoena question is: What if the subpoena isn't respected?" she said.

Hempowicz said she hopes that incoming Democratic leaders will try to work with the Trump administration before issuing subpoenas, arguing that forcing testimony could reduce voluntary transparency.

"There's been a lot of talk about the wisdom of Democrats starting the next Congress with a slew of subpoenas," she said.