Sen. Joe Manchin (D-W.Va.) said Thursday that the U.S. is on the "verge" of a constitutional crisis because of the forced resignation of Attorney General Jeff Sessions.

"I think it's a big mistake to let Sessions go," Manchin, who was the only Democratic senator to vote to confirm the former attorney general, said on "CBS This Morning."

Manchin pointed to the potential ramifications Sessions's ouster could have on special counsel Robert Mueller's investigation into Russia's election interference to back up his claim.

His comments came just a day after Sessions formally resigned from his role at the Department of Justice at President Trump's request. Trump announced on Twitter on Wednesday that Matthew Whitaker, Sessions's chief of staff, will serve as acting attorney general.

Whitaker, who has publicly criticized certain elements of the Mueller investigation, will now oversee it.

Deputy Attorney General Rod Rosenstein had been overseeing the probe since Sessions recused himself in early 2017.

Democratic lawmakers, including Manchin, have criticized Whitaker's oversight of the Mueller probe.

"What raises my concerns is a person that's been so vocal against the investigation that was going on is [put] in charge a day after the [midterm] election," Manchin told CBS. "I think that gives concern to every senator, Democrat and Republican. We are a country - the rule of law is everything.

"Looking like it's been tilted one way or the other is wrong."

Trump has repeatedly called the Russia investigation a "witch hunt," and on Wednesday said that he could fire everyone in Mueller's office if he wanted. He said he would not take that step for political reasons, however.