Sen. Richard Blumenthal (D-CT) said Wednesday that the resignation of Attorney General Jeff Sessions represents a “break the glass moment.”

This is a break the glass moment. Replacing the Attorney General with a non-Senate-confirmed political staffer is highly irregular & unacceptable,” tweeted Blumenthal. “Protecting the Special Counsel investigation is more urgent than ever.”

Earlier, President Donald Trump requested Attorney General Jeff Sessions resign as the country’s chief law enforcement officer. Trump announced in a tweet that he was naming Sessions’ chief of staff Matthew Whitaker, a former United States attorney from Iowa, as acting attorney general. Whitaker has previously criticized special counsel Robert Mueller’s investigation into potential Russian interference in the 2016 election.

Asked whether Whitaker would assume control over Mueller’s investigation, Justice Department spokeswoman Sarah Flores said Whitaker would be “in charge of all matters under the purview of the Department of Justice.” The Justice Department did not announce a departure for Deputy Attorney General Rod Rosenstein, who appointed Mueller more than a year and a half ago and has closely overseen his work since then.

Sessions told the president in a one-page letter that he was submitting his resignation “at your request.”

Whitaker once opined about a situation in which Trump could fire Sessions and then appoint an acting attorney general who could stifle the funding of Mueller’s probe.

“So I could see a scenario where Jeff Sessions is replaced with a recess appointment and that attorney general doesn’t fire Bob Mueller, but he just reduces his budget to so low that his investigation grinds to almost a halt,” Whitaker said during an interview with CNN in July 2017.

In an op-ed published via CNN, Whitaker wrote: “Mueller has come up to a red line in the Russia 2016 election-meddling investigation that he is dangerously close to crossing.”

This is not the first time Blumenthal has said the president firing Justice Department officials overseeing the Mueller probe would signify a “break the glass” moment. The Connecticut Democrat told Yale Law School in September that Rosenstein’s removal from the investigation would represent such circumstances.

The Associated Press contributed to this report.