Former House Speaker Newt Gingrich has grown critical of special counsel Robert Mueller's ability to conduct a fair investigation into Russia's meddling in the 2016 special election.

"Republicans are delusional if they think the special counsel is going to be fair. Look who he is hiring.check fec reports. Time to rethink," Gingrich tweeted early Monday.

Less than a month ago, Gingrich praised Mueller as a "superb choice" to lead the investigation.

"Robert Mueller is superb choice to be special counsel. His reputation is impeccable for honesty and integrity. Media should now calm down," Gingrich, a staunch supporter of President Trump, wrote in May.

Deputy Attorney General Rod Rosenstein appointed Mueller, a former FBI director, after James Comey's memos on his private conversations with Trump leaked to the press. Mueller and Comey have been longtime friends.

On Sunday, Gingrich accused Comey of having "tainted" the special counsel after the former FBI director admitted to leaking the memo after he was fired in an effort to get the Justice Department to appoint a special counsel in the Russia investigation.

"Comey himself, by his own testimony, tainted this particular process. You have a director of the FBI deliberately leaking in order to create a special counsel, who we're now supposed to believe is going to be this neutral figure. I think that's just nonsense," Gingrich said.