Mitt Romney hasn’t even been sworn in as a senator, but he had strong words Wednesday over concerns that acting Attorney General Matt Whitaker would pull the plug on the Mueller investigation.

“It is imperative that the important work of the Justice Department continues, and that the Mueller investigation proceeds to its conclusion unimpeded.”

Sen.-elect Mitt Romney

Along with that warning to Whitaker, the onetime Republican presidential candidate, who won a U.S. Senate seat in Utah on Tuesday, also thanked Jeff Sessions for his service in a tweet late Wednesday.

Read: Attorney General Jeff Sessions’s resignation letter reveals Trump told him to step down

The ouster of Sessions, which also removes Deputy Attorney General Rod Rosenstein from overseeing Mueller’s investigation into Russian interference in the 2016 election and potential collusion by the Trump campaign, caused alarm from many Democratic lawmakers, especially in the wake of an opinion column written by Whitaker last year, in which he called for limits to Mueller’s probe.

And Romney wasn’t the only Republican senator to share that concern.

“It is imperative that the Administration not impede the Mueller investigation,” Sen. Susan Collins of Maine tweeted Wednesday. “I’m concerned Rod Rosenstein will no longer be overseeing the probe. Special Counsel Mueller must be allowed to complete his work without interference—regardless of who is AG.”

Sen. Jeff Flake of Arizona, who is outgoing but still has his seat until the new Senate is sworn in on Jan. 3, called for the full Senate to pass a bill protecting Mueller’s investigation.

“Earlier this year, we passed S.2644, the Special Counsel Independence and Integrity Act, out of the Senate Judiciary Committee,” he tweeted. “The bill would safeguard Robert Mueller’s investigation. Leader McConnell should bring the bill to the Senate floor as soon as possible.”