Retiring 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.) on Sunday said the crowds at rallies for President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE and other Republicans reflect "spasms of a dying party," adding that the GOP will have to formulate a governing agenda to reach more voters.

"When you look at some of the audiences cheering for Republicans, sometimes, you look out there and you say, 'those are the spasms of a dying party,' " Flake said on ABC's "This Week."

"When you look at the lack of diversity, sometimes, and it depends on where you are, obviously, but by and large, we're appealing to older white men and there are just a limited number of them, and anger and resentment are not a governing philosophy," he continued.

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Flake, who has announced he will not seek reelection in 2018, has been a frequent critic of Trump, earning the president's ire on Twitter.

The Arizona senator refused to back Republican Alabama Senate candidate Roy Moore Roy Stewart MooreRoy Moore sues Alabama over COVID-19 restrictions Vulnerable Senate Democrat urges unity: 'Not about what side of the aisle we're on' Sessions hits back at Trump days ahead of Alabama Senate runoff MORE, calling him unfit for office. He wrote a $100 check to Democrat Doug Jones in the race, writing "Country over Party" in the memo line.

Flake on Sunday said he believes Moore, who earned Trump's full endorsement and had the backing of former White House chief strategist Stephen Bannon, will be a "lasting" stain on the Republican Party.

Flake also said he's hopeful Bannon's role within the party is being marginalized.

"The last thing we need is to push that ultranationalist, ethno-nationalist, protectionist kind of element of the party. That's not good for us," Flake said.