Maine Sen. Angus King said Sunday that President Trump’s criticism of special counsel Robert Mueller’s Russia investigation is a huge mistake and that any attempt by the president to have Mueller fired would trigger a constitutional crisis.

King, speaking from Carrabassett Valley, made his remarks on the CBS News program “Face the Nation,” whose moderator, Margaret Brennan, asked him about the latest personnel changes at the CIA and a firing at the FBI.

Earlier Sunday, Trump fired off angry tweets about Mueller’s investigation. The president also attacked former FBI Director James Comey and former Deputy Director Andrew McCabe, who was fired Friday night hours before his retirement was to have taken effect and he would have been eligible for full retirement benefits. King said the timing of McCabe’s termination seemed “mean-spirited.”

The president’s tweets followed a call Saturday for an end to the Mueller investigation by John Dowd, Trump’s personal lawyer.

King, an independent who caucuses with the Democrats, sits on the Senate Intelligence Committee, which has been conducting its own investigation into Russian interference in the 2016 presidential election.

Brennan asked him about McCabe’s firing by Attorney General Jeff Sessions, Trump’s complaints about the Mueller probe, and also about Gina Haspel, the president’s nominee to head the CIA.

King said he wants to see the Justice Department inspector general’s report before he draws any conclusions about the McCabe firing. Sessions said the report concluded that McCabe had made “unauthorized disclosure to the news media and lacked candor – including under oath – on multiple occasions.” McCabe has denied the conclusions.

“But even if you assume for the moment that the report said he did these – he had these violations of code of the FBI, if you will – my problem is the timing and the way it all worked. Just seems mean-spirited to come down on a guy within 48 hours of his scheduled retirement. He had 21 years of exceptional service in the FBI. So it was clearly rushed,” King said.

Asked about Trump’s tweets against Mueller, King said it is a big mistake for the president and dangerous for the country. He said Mueller is as straight an arrow as there is in America, a former Marine, and a Republican.

“For the administration to keep trying to undercut what they’re trying to do, the president keeps saying there’s no story here, they didn’t do anything wrong. If they didn’t do anything wrong, why are they going to such extreme lengths to undermine this investigation, which is being carried out in a very responsible way?” King said.

He said the fact that the investigation has already resulted in three or four guilty pleas and 15 to 20 indictments tells him “there is something going on here, and there’s something serious.”

King said his top question for Haspel – currently the CIA’s deputy director – when she comes before his committee will be about her role in the agency’s harsh interrogation of terrorism suspects in the aftermath of the Sept. 11 attacks and the destruction of videotapes depicting those interrogation techniques, which some have called torture. He said he wants to know how Haspel feels about the torture that went on, what her view is looking back, and what she would do if Trump ordered a resumption of those techniques.

“Although it is the law of the land that it can’t be done now, you know, is he going to try to change that, is she going to follow orders from a president that tells her to do something that’s contrary to the law because she was involved in this project 15 years ago?” King said.