Corey Lewandowski, U.S. President Donald Trump's former campaign manager and close confidant, on Tuesday vigorously defended his former boss and lashed out at Democrats but repeatedly avoided answering their questions during a chaotic hearing before a congressional panel considering whether to impeach Trump.

Lewandowski, who is considering running for a U.S. Senate seat from New Hampshire, tangled with Democratic members of the House of Representatives judiciary committee and told the panel he would refuse to answer any questions about his conversations with Trump. The president's fellow Republicans unsuccessfully sought to have the hearing adjourned shortly after it began.

Lewandowski was the first impeachment witness to appear before the committee since former special counsel Robert Mueller testified in July about his inquiry that detailed Russian 2016 election interference and Trump's actions to impede the investigation.

"We as a nation would be better served if elected officials like you concentrated your efforts to combat the true crises facing our country as opposed to going down rabbit holes like this hearing," Lewandowski said, in his opening remarks to the Democratic-led House judiciary committee.

The White House on Monday told Lewandowski not to discuss conversations he had with Trump after he became president including an exchange that Democrats view as evidence that Trump committed obstruction of justice by trying to interfere in a federal investigation and may need to be impeached.

White House lawyers sat behind Lewandowski during the hearing.

'An absolute coverup'

House judiciary committee chairman Jerrold Nadler, a Democrat, began the hearing by slamming the White House's legal team for instructing Lewandowski to limit the scope of his testimony by invoking the doctrine of executive privilege.

"We should call this what it is: an absolute coverup by the White House," Nadler said.

"The White House is advancing a new and dangerous theory: the crony privilege," Nadler added. "Where are the limits?"

Lewandowski repeatedly avoided answering questions from Nadler and other Democrats, citing the White House's instructions that he not discuss post-election interactions with Trump beyond those already detailed in Mueller's report on Russian interference in the election.

House judiciary committee chair Rep. Jerrold Nadler of New York gives an opening statement at the hearing on Tuesday. (Jacquelyn Martin/Associated Press)

Lewandowski sparred with Democratic Rep. Sheila Jackson Lee, accusing her of delivering a "rant." Jackson Lee said he was avoiding questions and interrupting her. Lewandowski faced friendlier questions from Republican lawmakers.

Lewandowski assailed the federal investigation into Russian interference in the 2016 election.

"It is now clear the investigation was populated by many Trump haters who had their own agenda — to try and take down a duly elected president of the United States. As for actual collusion or conspiracy, there was none. What there has been, however, is harassment of the president from the day he won the election," Lewandowski added.

Trump said on Twitter that Lewandowski had given "such a beautiful" opening statement. Trump in August sought to boost Lewandowski's potential Senate bid, calling his former aide "a fantastic guy" who would make a "great senator" and that "I like everything about him."

Such a beautiful Opening Statement by Corey Lewandowski! Thank you Corey! <a href="https://twitter.com/CLewandowski_?ref_src=twsrc%5Etfw">@CLewandowski_</a> —@realDonaldTrump

"This isn't a campaign rally. This is the first hearing where you can tell the American people how you participated in the president's effort to obstruct justice," Democratic Rep. David Cicilline wrote on Twitter.

Democrats asked Lewandowski about the president's effort to persuade then-attorney general Jeff Sessions to redirect the Mueller probe away from the 2016 Trump election campaign. The episode is among a number of incidents contained in Mueller's 448-page investigative report made public in April that Democrats view as evidence that Trump obstructed justice.

Democratic Rep. Steve Cohen asked Lewandowski, "You didn't think that it was illegal to obstruct justice?"

"The president never asked me to do anything illegal," Lewandowski said.

White House told ex-aides not to testify

Under the U.S. Constitution, the House has the power to vote to impeach a president while the Senate then would hold a trial on whether to remove him from office. The House is controlled by Democrats and the Senate by Trump's fellow Republicans.

White House counsel Pat Cipollone told the committee in a letter Monday that Lewandowski could not testify about conversations with Trump after he became president or with his senior advisers.

The White House also directed two other witnesses, former Trump White House aides Rob Porter and Rick Dearborn, not to testify. Cipollone's letter said they were "absolutely immune from compelled congressional testimony with respect to matters related to their service as senior advisers to the president."

In June 2017, Trump met Lewandowski, then a private citizen, at the White House and dictated a message he was to deliver to Sessions. The message said Sessions should shift the Russia probe's focus to future elections despite his recusal from the investigation.

At a second meeting a month later, Trump asked about the status of the message and said Lewandowski should "tell Sessions he was fired" if he would not meet with Lewandowski, according to the Mueller report.

Trump fired Lewandowski as his campaign manager in June 2016 but the two remained close. During the campaign, Lewandowski had often generated controversy including when he was charged with misdemeanor battery after being accused of forcefully grabbing a female reporter in Florida. The charge was later dropped.