President Donald Trump's new acting Justice Department chief Matthew Whitaker drew tough criticism Friday over his denial of Russia's meddling in the 2016 elections and his past involvement in a company fined millions for bilking clients.

Just two days after being named acting attorney general, the former federal prosecutor and TV commentator faced questions over his ability to fairly supervise Special Counsel Robert Mueller's investigation into possible collusion between Trump's campaign and Russia, given Whitaker's proposal last year to starve Mueller of funding.

Questions were also being asked about his role on the board of a patent marketing company fined $25 million last May to settle fraud allegations.

Late Friday, the Wall Street Journal said the FBI - which is now under Whitaker - is conducting a criminal investigation of the company, World Patent Marketing Inc.

And members of Congress said his appointment was unconstitutional, because he has never been confirmed for top office by the US Senate.

"As an unconfirmed political appointee, Mr Whitaker has not been subject to the scrutiny that the Constitution requires to ensure that he has the character, integrity and ability to fulfil the grave responsibilities of this job," top Senate Democrat Chuck Schumer said in a letter addressed to Trump Friday.

"These concerns are heightened by specific expressions of bias against the special counsel investigation that Mr Whitaker made just last year," Schumer said.

Whitaker was named to the powerful job Wednesday after Trump fired attorney general Jeff Sessions.

His appointment was greeted by protests in different cities calling for the protection of the Mueller probe, which potentially threatens the president and his family.