President Trump escalated tensions with his former top White House lawyer on Thursday, sharply questioning the credibility of one of the special counsel’s key witnesses as congressional Democrats seek his testimony.

In a morning tweet, Trump disputed that he had told Donald McGahn, then White House counsel, to pursue the firing of special counsel Robert S. Mueller III amid his investigation into Russian interference in the 2016 election.

That episode and others are included in a report released by Mueller last week that have prompted House Democrats to issue a subpoena for McGahn as they examine whether Trump sought to obstruct Mueller’s efforts.

“I never told then White House Counsel Don McGahn to fire Robert Mueller, even though I had the legal right to do so,” Trump tweeted Thursday. “If I wanted to fire Mueller, I didn’t need McGahn to do it, I could have done it myself. Nevertheless, Mueller was NOT fired and was respectfully allowed to finish his work on what I, and many others, say was an illegal investigation.”

Since Robert S. Mueller III was appointed in May 2017 to investigate the Trump campaign's ties to Russia, President Trump has relentlessly attacked him. (Jenny Starrs/The Washington Post)

The president’s attacks on his former White House counsel are driven by Trump’s growing belief that his opponents on Capitol Hill will use McGahn’s testimony as the cornerstone of a possible impeachment case against him, according to two White House officials who spoke on the condition of anonymity because they were not authorized to speak publicly.

“He’s frustrated because he knows the Democrats are going to jump on this and use it as a weapon, so he’s fighting back,” one of the officials said.

Trump’s latest round of critical comments about McGahn came as his personal legal team, led by Rudolph W. Giuliani, has spent days assailing McGahn’s version of events documented in Mueller’s report.

McGahn did not respond to Trump on Thursday and is eager to avoid a dramatic showdown with his former boss, according to an ally, who spoke on the condition of anonymity because they were not authorized to speak publicly. McGahn is “shrugging off” the tweets and has described them to associates as part of Trump’s raging political battle with House Democrats rather than a personal battle with him, the ally said.

Still, McGahn’s side has not been dormant as Trump’s lawyers — and now the president — have questioned McGahn. And some Trump advisers said privately Thursday that they fear Trump’s ire could eventually prompt McGahn to speak to protect his reputation, potentially creating a wave of new challenges for the White House.

McGahn’s attorney William A. Burck issued a statement last week in response to critiques from Giuliani, who called McGahn “hopelessly confused” on CNN. Burck called Giuliani’s remarks a “mystery” and said the incidents in the report were “accurately described.” Burck declined to comment Thursday.

McGahn emerged as a central witness in Mueller’s 448-page report, offering hours of interviews about several occasions when Trump ordered him to do “crazy” things, according to the special counsel’s findings. That included trying to persuade the Justice Department to get rid of Mueller, according to the report.

McGahn did not try to do so and almost resigned over the episode. He also refused a request from Trump to issue a public statement denying reports Trump wanted to fire Mueller, the report says.

Trump and White House lawyers are moving toward asserting executive privilege to prevent McGahn from testifying about his recollections in front of the House Judiciary Committee. It is one of many areas of inquiry where the administration is working to halt congressional oversight requests.

White House Counsel Pat Cipollone and the president’s lawyers are preparing for an extensive legal battle, if necessary, over subpoenas from Congress, the officials said.

In a Tuesday interview with The Washington Post, Trump maintained that the White House Counsel’s Office had not “made a final, final decision” on executive privilege but that he opposes cooperation with House Democrats.

“There is no reason to go any further, and especially in Congress where it’s very partisan — obviously very partisan,” Trump said.

The standoff could be one of the most significant debates over presidential privilege since Richard Nixon sought to block the release of his White House tapes in the Watergate investigation.

The House Judiciary Committee issued a subpoena Monday ordering McGahn to testify before the committee next month and hand over records pertaining to federal investigations of Trump, his finances, his campaign and charges that he sought to obstruct justice.

In recent television interviews, Giuliani and former Trump lawyer John Dowd have cast doubt on McGahn’s recollection of Trump’s alleged order to seek Mueller’s firing. Both argued that Trump wasn’t as direct as McGahn believed. Trump was seeking only to have Mueller “vetted,” Dowd said during an appearance on Fox News.

Mueller reported two calls in which McGahn said Trump “directed him” to tell Deputy Attorney General Rod J. Rosenstein that Mueller should be removed from his position.

The first call took place in June 2017 after The Post reported that Mueller was investigating whether Trump had obstructed justice as part of the Russia probe. McGahn recalled that Trump said something like, “You gotta do this. You gotta call Rod,” according to Mueller’s report.

Trump was more direct in a second call, saying something along the lines of, “Call Rod, tell Rod that Mueller has conflicts and can’t be the Special Counsel” and “Mueller has to go,” according to the report, which documented McGahn’s recollections.

Mueller’s report did not find sufficient evidence to bring charges of criminal conspiracy with Russia against Trump or anyone associated with his campaign, and did not offer a conclusion about whether Trump obstructed justice. Attorney General William P. Barr later concluded that there was not sufficient evidence for obstruction-of-justice charges.

Trump and McGahn had an adversarial relationship at times during his tenure in the White House, with McGahn contemplating quitting at various times. But he was also deeply involved in some of the president’s primary accomplishments, including the confirmation of two Supreme Court judges and a record number of federal judiciary appointments.

Karoun Demirjian, Carol D. Leonnig and Josh Dawsey contributed to this report.