The top ethics lawyer to former President George W. Bush warned on Saturday that if President Trump fires special counsel Robert Mueller, Vice President Pence "will soon become the 46th President."

"Just appeared on FOX News. They are building a case for firing Mueller. If that happens Mike Pence Michael (Mike) Richard PencePence vows for law and order everywhere Trump met with chants of protest as he pays respects to Ruth Bader Ginsburg The Hill's 12:30 Report: Ginsburg lies in repose MORE will soon become the 46th President," former White House ethics lawyer Richard Painter wrote on Twitter.

Just appeared on FOX News. They are building a case for firing Mueller. If that happens Mike Pence will soon become the 46th President. — Richard W. Painter (@RWPUSA) June 17, 2017

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Painter's comments come as Trump and his allies cast doubt on Mueller's ability to conduct a fair investigation into Russia's role in the 2016 election and possible collusion between the Trump campaign and Russia.

Mueller was appointed last month as special counsel to lead the probe, after Trump abruptly fired FBI Director James Comey.

The appointment was hailed by partisans on both sides of the aisle, who praised Mueller as a model law enforcement official and an aggressive, yet fair, investigator.

But some Republicans have appeared to sour on Mueller in recent days, arguing that his friendship with Comey and decision to hire prosecutors who have donated to Democratic candidates gave the appearance of conflicts.

Trump blasted Mueller on Twitter on Thursday, saying the Russia probe was being "led by some very bad and conflicted people," apparently referring to the special counsel.

That tweet came a day after The Washington Post reported that Mueller had expanded his probe to look into whether Trump himself tried to obstruct justice by allegedly pressing top intelligence officials to intervene in ongoing investigations.