Sen. Joe Manchin III (D-W.Va.) speaks to supporters after being re-elected on Tuesday. (Tyler Evert/AP)

Sen. Joe Manchin III (W.Va.), the only Democrat who voted to confirm Jeff Sessions as attorney general, said Thursday that President Trump’s decision to force him out of office was “a big mistake” and the country is now on the verge of a constitutional crisis.

Manchin’s comments came a day after Sessions resigned at Trump’s request and was replaced by his chief of staff, Matthew G. Whitaker. The new acting attorney general has been openly critical of the investigation by special counsel Robert S. Mueller III — prompting concerns that he could curtail the probe.

Asked during an appearance on “CBS This Morning” if he thinks Trump’s move has brought the country to a constitutional crisis, Manchin said: “I think we’re on the verge of that.”

Manchin advocated passage of a bill that would protect the work of Mueller, who is investigating possible coordination between Russia and the Trump campaign in the 2016 election and potential obstruction of the probe by Trump.

Manchin said that while Sessions, a former Republican senator from Alabama, was more conservative than he is, he showed integrity as attorney general.

“The rule of law meant everything to Jeff, I knew that, and I think he stuck with that,” Manchin said. “I think it’s a big mistake to let Jeff Sessions go.”

Manchin, who was reelected Tuesday in a state where Trump remains popular, also voiced concern about Whitaker’s past criticism of the Mueller investigation before taking the job as Sessions’s chief of staff at the Justice Department.

“What raises my concerns is a person that’s been so vocal against the investigation that was going on now putting in charge a day after the election,” Manchin said. “I think that sheds bad light on it. I think that gives concern to every senator, Democrat and Republican. . . . Looking like it’s been tilted one way or another is wrong.”