Donald Trump's ex-campaign director Cory Lewandowsk offered a fierce defence of his former boss.

Donald Trump's ex-campaign director Cory Lewandowski offered a fierce defense of his former boss Monday in a contentious congressional hearing, the first under new Democratic guidelines governing a presidential impeachment investigation.

Lewandowski's defiance before the House Judiciary Committee frustrated Democrats, who sought to pin him down on questions about potential obstruction of justice and special counsel Robert Mueller's report on 2016 Russia election interference.

Lewandowski, who worked for Trump during the campaign but not in the White House, slammed the Russia probe as being "populated by many Trump haters who had their own agenda: to take down a duly elected president of the United States."

"As for actual collusion or conspiracy, there was none," he added. "What there has been, however, is harassment of the president from the day he won the election."

The performance antagonized Democrats but was immediately noticed by perhaps the most important person watching, Trump himself, who tweeted: "Such a beautiful Opening Statement by Corey Lewandowski! Thank you Corey!"

Lewandowski's appearance was the first before Congress by a major figure in Trump's orbit under new committee rules as the panel considers whether to lay out articles of impeachment against Trump.

Democrats quizzed Lewandowski about potential obstruction of justice episodes that were detailed in Mueller's report, which said the president personally asked Lewandowski to urge then-attorney general Jeff Sessions to quash the Mueller probe investigating the 2016 election.

They were stymied, as Lewandowski repeatedly stated he was honoring the White House's order not to discuss certain communications.

"The White House has directed that I not disclose the substance of any discussions with the president," he said.

Exasperated Democrats were left with few concrete answers.

"You didn't think that was illegal to obstruct justice?" congressman Steve Cohen asked, referring to Trump's request for Lewandowski to lean on Sessions.

"Congressman, the president didn't ask me to do anything illegal," Lewandowski responded.

Two other Trump aides -- Rick Dearborn, a former Trump campaign advisor and former White House deputy chief of staff, and Robert Porter, an ex-White House staff secretary -- were also scheduled to testify Tuesday.

But the White House blocked their appearances, arguing in a letter to committee chairman Jerry Nadler that the two are "absolutely immune" from having to testify.

"I think we should call this what it is: an absolute cover-up by the White House," Nadler said.