Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeHow fast population growth made Arizona a swing state Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden MORE (R-Ariz.) slammed President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE as unmoored from conservative principles and said he hopes he gets a primary opponent in 2020 who can remind Republicans of "what it means to be decent."

“I do hope that somebody does run in the primary against the president. I think the Republicans need to be reminded of what conservatism really is and what it means to be decent, and we haven’t had that kind of politics lately,” Flake, who is retiring after his term ends in January, told C-SPAN.

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Flake said the ceremonies honoring the late Sen. John McCain John Sidney McCainThe Memo: Trump's strengths complicate election picture Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' Cindy McCain: Trump allegedly calling war dead 'losers' was 'pretty much' last straw before Biden endorsement MORE (R-Ariz.) after his death from brain cancer in August served as a “reminder” of the stark differences between his politics and those of the president.

“That whole week commemoration of his life and his politics was just an additional reminder of stark differences that there are sometimes in politicians,” he said about the period after McCain's death.

“I think we as a Republican Party certainly have got to get back to a kind of decency that has characterized the party for a while,” he added.

Flake said that while he doesn’t expect the president, who is 72 years old, to change, he is concerned about how he has transformed rhetoric on the right among voters.

“When you see him at a rally, the disturbing thing isn’t so much what he says anymore, it’s the cheers from people behind him and the chants of ‘lock her up’ for example, that’s just unseemly, and it does make me fear that it’s going to be a longer process to get out of this than it should be. But we will, we have to. Anger and resentment are not a governing philosophy,” he said.

Flake, who's seen as potentially considering a presidential run himself, told C-SPAN that whoever runs against the president as a Republican would have large obstacles to overcome.

“It’s the entire machine. This is the president’s party right now, no doubt. To win a Republican primary around the country, you have to really embrace the president’s policies and condone his behavior, and that’s the bottom line right now. But it won’t always be that way, and over time we’ll realize where we need to go,” he said.

Flake has been a frequent critic of Trump’s character since the 2016 presidential election. He will retire at the end of his term after saying he couldn’t win a Republican primary in Arizona in the current political environment because voters would not tolerate that criticism.

Ohio Governor John Kasich, who ran for president in 2016, has also been floated as a potential Trump opponent in 2020.