Top Democrats have stepped up pressure on Acting US Attorney-General Matthew Whitaker to step aside from overseeing a special counsel probe into Russian meddling in the 2016 US presidential election, vowing to order him to testify early next year.

Key points: Matthew Whitaker's appointment "an attack" on Mueller investigation, Democrats say

Matthew Whitaker's appointment "an attack" on Mueller investigation, Democrats say Future of probe into Russian meddling in 2016 election in question after Sessions' sacking

Future of probe into Russian meddling in 2016 election in question after Sessions' sacking Democrats say they will attempt to introduce legislation to protect the investigation

Representative Jerrold Nadler, the expected incoming chairman of the House Judiciary Committee, said the committee plans to subpoena Mr Whitaker to testify next year as its first witness.

"His appointment is simply part of an attack on the investigation by Robert Mueller," Mr Nadler said on the US ABC News show This Week.

"He should recuse himself. He has expressed total hostility to the investigation."

Senate Democratic leader Chuck Schumer said he would seek to tie a measure protecting special counsel Robert Mueller to must-pass legislation if Mr Whitaker does not recuse himself from oversight of the probe.

Chuck Schumer said keeping Mr Whitaker in charge of the Mueller investigation would create a "constitutional crisis." ( Senate Democrats/flickr.com/CC-BY-2.0 )

"Democrats in the House and Senate are going to attempt to put on must-pass legislation, mainly the spending bill, legislation that would prevent Whitaker from interfering in any way with the Mueller investigation," Mr Schumer told The Associated Press.

During the interview, Senator Schumer said keeping Mr Whitaker in charge of the investigation would create a "constitutional crisis" and said if he doesn't recuse himself, Democrats would push to introduce legislation to protect Mr Mueller's investigation.

In a letter to the Justice Department's chief ethics officer, Senator Schumer, House Democratic leader Nancy Pelosi and other top Democrats advised Mr Whitaker to recuse himself from overseeing Special Counsel Mr Mueller's probe.

"Allowing a vocal opponent of the investigation to oversee it will severely undermine public confidence in the Justice Department's work on this critically important matter," the letter said.

Nancy Pelosi said Democrats may cede to some Republican requests to pass DACA legislation. ( Reuters: Joshua Roberts )

Democrats have increasingly expressed alarm since last week, when President Donald Trump ordered attorney-general Jeff Sessions to resign and replaced him with Mr Whitaker, Mr Sessions' chief of staff.

Democrats have also questioned if Mr Whitaker was properly appointed to the position.

"If he doesn't recuse himself, if he has any involvement whatsoever in this Russia probe, we are going to find out whether he made commitments to the President about the probe, whether he is serving as a back channel to the President or his lawyers about the probe, whether he is doing anything to interfere with the probe," said Adam Schiff, presumed chair of the House Intelligence Committee, on NBC's Meet the Press.

US Attorney-General Jeff Sessions testifies before the Senate Intelligence Committee. ( Reuters: Jonathan Ernst )

Mr Sessions' removal paved the way for Mr Whitaker to take over the oversight of Mr Mueller's investigation from Deputy Attorney-General Rod Rosenstein, who appointed Mr Mueller in May 2017 after Mr Sessions recused himself from the probe.

Prior to working at the Justice Department, Mr Whitaker made multiple negative comments about Mr Mueller's investigation and its scope.

In addition, Mr Whitaker is also a close friend of Mr Trump's 2016 election campaign co-chair Sam Clovis, who has since become a witness in Mr Mueller's investigation.

"Mr Whitaker's relationship with Mr Clovis, who is a grand jury witness in the special counsel investigation, as well as Mr Whitaker's other entanglements, raise additional concerns about his ability to supervise the investigation independently and impartially," the letter said.

Kellyanne Conway made the comments during a Fox News interview from the White House. ( AP: Fox News )

Kellyanne Conway, a counsellor to Mr Trump, defended Mr Whitaker's oversight of the probe when asked about it on This Week.

"Comments that Matt Whitaker made as a private citizen on cable TV does not disqualify him from being fair and impartial by overseeing this investigation," she said.

She added that Mr Trump is "100 per cent behind Matt Whitaker".

Reuters/AP