(CNN)There has been a storyline kicking around of late that Donald Trump is losing sway within the Republican Party, noting that his Senate picks in places like Alaska and North Carolina appear to be underperforming.

       While that may be true in specific circumstances, Trump's death grip on the GOP more broadly doesn't appear to be relaxing much.

       The latest evidence of that reality came Tuesday morning when Michigan Rep. Fred Upton announced he would not run for a 19th term against fellow Republican Rep. Bill Huizenga this year.

       "Even the best stories have a last chapter," Upton said on the House floor. "This is it for me."

       While Upton made no mention of Trump, it's impossible to avoid seeing his decision to retire through that lens.

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       Upton had angered Trump by being one of 10 House Republicans who voted to impeach the then-President for his role in the January 6 riot at the US Capitol. Trump also endorsed Huizenga last month.

       With his announcement, Upton is now the fourth of those 10 members to opt against seeking reelection, joining Reps. John Katko of New York, Adam Kinzinger of Illinois and Anthony Gonzalez of Ohio on the sidelines.

       "UPTON QUITS! 4 down and 6 to go. Others losing badly, who's next?" Trump said in a statement Tuesday.

       Among the other six, Trump has endorsed five of their GOP primary challengers.

       * Wyoming Rep. Liz Cheney: Trump has endorsed Harriet Hageman.

       * Michigan Rep. Peter Meijer: Trump has endorsed John Gibbs.

       * Washington Rep. Jaime Herrera Beutler: Trump has endorsed Joe Kent.

       * South Carolina Rep. Tom Rice: Trump has endorsed Russell Fry.

       * Washington Rep. Dan Newhouse: Trump has endorsed Loren Culp.

       The lone Republican who voted to impeach Trump who the former President hasn't endorsed against is California Rep. David Valadao, a close ally of House Minority Leader Kevin McCarthy. (The filing deadline for congressional candidates in California was March 11.)

       Trump has closely tracked the political fates of these 10 members, celebrating each time one retires. "Great news, another one bites the dust," Trump said in January following the news of Katko's retirement.

       There is no question then that Trump is aggressively trying to purge the Republican Party of any voices of dissent. (He's been doing this for a while; Trump helped push Arizona Sen. Jeff Flake out of office way back in 2017.)

       And there is also no question it's working. It was hard to see how the likes of Gonzalez or Kinzinger or Upton could win reelection with Trump -- and the GOP base he commands -- standing against them.

       Upton's retirement should serve as a reminder to any who doubted: This is still Donald Trump's party. Bigly.