White House opens new front in Trump's battle to block congressional investigations
US President Donald Trump's former communications director Hope Hicks (r) has been ordered not to hand over documents to a House committee investigating the president (AFP Photo/Mandel NGAN)

The White House has told former top presidential aide Hope Hicks and another former senior staffer to not cooperate with a congressional investigation of alleged obstruction of justice by Donald Trump, a senior Democrat said Tuesday.

House Judiciary Committee Chairman Jerry Nadler said Hicks and Annie Donaldson, the former chief of staff to White House counsel Don McGahn, were instructed to not comply with his panel's subpoena for documents related to their time in the White House.

The move opened a new front in Trump's battle to block congressional investigations of him, with a number of Democrats calling for impeachment of the president over acts of obstruction documented in Special Counsel Robert Mueller's Russia meddling investigation.

It came two weeks after Donaldson's former boss, White House counsel Don McGahn, ignored a subpoena from Nadler's committee to testify on Trump's order.

"The president has no lawful basis for preventing these witnesses from complying with our request," Nadler said.

Nadler's committee is investigating alleged obstruction of justice, corruption, and other possible abuses by Trump, his associates, and members of his administration.

His committee would be responsible for any impeachment effort in the House.

The three former staffers provided key testimony supporting Mueller's documenting multiple acts of possible criminal obstruction by Trump in the Russia investigation.

Mueller did not declare any charges against Trump, citing Justice Department restrictions, but said it was Congress's responsibility to hold the president to the law.

Nadler said the committee will continue to press for the documents and still expect Hicks and Donaldson to testify.

On Monday, Democratic Representative Steny Hoyer, the House Majority Leader, said he intends to introduce a resolution to find McGahn and Attorney General Bill Barr in contempt of Congress for refusing to comply with subpoenas for testimony and documents related to the Russia and obstruction investigations.

The resolution, to be submitted next week, will authorize the Judiciary Committee to go to court to enforce its subpoenas.

"This administration's systematic refusal to provide Congress with answers and cooperate with congressional subpoenas is the biggest cover-up in American history, and Congress has a responsibility to provide oversight on behalf of the American people," Hoyer said.

Separately, House Oversight Committee Chairman Elijah Cummings threatened contempt citations against Barr and Secretary of Commerce Wilbur Ross for defying subpoenas seeking information related to the alleged politicization of the US census.