WASHINGTON (Reuters) - Democrats in the U.S. House of Representatives are locked in a dispute with Acting Attorney General Matthew Whitaker over scheduling a date for him to testify to lawmakers amid concerns about his appointment by President Donald Trump.

FILE PHOTO: Acting U.S. Attorney General Matthew Whitaker climbs the steps of Air Force One to depart with President Donald Trump for travel to Kansas City, Missouri from Joint Base Andrews, Maryland, U.S., December 7, 2018. REUTERS/Jonathan Ernst/File Photo

House Judiciary Committee Chairman Jerrold Nadler said after Democrats won the House majority in November’s elections that Whitaker would be his first witness.

Some Democrats are concerned that Whitaker’s appointment to replace ousted Attorney General Jeff Sessions violated the U.S. Constitution and represented an effort to undermine Special Counsel Robert Mueller’s investigation into Russian interference in the 2016 U.S. election and possible coordination with Trump campaign members. Trump denies campaign collusion with Moscow.

According to Nadler, Whitaker agreed in November that he would appear for a hearing in January. But Nadler said on Wednesday that the Department of Justice was now trying to cite the partial government shutdown as a reason to delay Whitaker’s testimony until Feb. 12 or Feb. 13, telling the committee he would need “at least two weeks removed from a partial government shutdown” before he could appear.

“I cannot accept your proposal,” Nadler wrote in a letter to Whitaker.

“We are willing to work with you to identify a mutually identifiable date for your testimony, but we will not allow that date to slip past January 29, 2019 - the day of the President’s scheduled address to Congress, when we know you will be in Washington.”

A Justice Department spokeswoman did not immediately reply to a request for comment on Nadler’s letter.

For the House Judiciary Committee, time could be of the essence.

It is possible that by mid-February, Trump’s nominee for Attorney General, William Barr, could be a confirmed by the U.S. Senate, making Whitaker’s testimony less pressing.

Barr’s confirmation hearing before the Senate Judiciary Committee is scheduled for Jan. 15 and 16.

Whitaker’s appointment on Nov. 7 as acting attorney general is now the subject of multiple lawsuits. He is also under scrutiny for his decision not to recuse himself from overseeing Mueller’s probe into Russian meddling, disregarding advice from ethics officials who found his participation could arguably create the appearance of a conflict.

Whitaker has expressed scepticism about the Mueller investigation, which is overseen by another official, Rod Rosenstein, who is preparing to leave his job soon after a new attorney general takes office.

“The public is entitled to know why you chose to disregard the advice of career ethics officials at the Department with respect to your oversight of the Special Counsel,” Nadler’s letter said.