Today's Kentucky primary is President Trump's biggest test yet of whether his iron grip on the Republican base can hold even as war and inflation batter his national standing.Why it matters: Trump is trying to take out longtime antagonist Thomas Massie in the most expensive House primary in history — a contest that will show whether his iconoclastic brand of libertarian politics still has a place in the GOP. Until now, the seven-term Massie had never faced a serious threat to reelection. Trump changed that.Trump's political operation launched an aggressive effort to unseat Massie last year, its first such effort to defeat a sitting Republican incumbent.The fight between Massie and Trump-backed rival Ed Gallrein has drawn more than $32 million in ad spending, according to AdImpact.Driving the news: Trump has spent months portraying Massie as disloyal to the MAGA movement. He's called Massie a "moron," a "nut job" and "major Sleazebag.""He is the Worst "Republican" Congressman in History," Trump posted on Truth Social Sunday.At a March rally in Massie's district, Trump said: "Give me somebody with a warm body to beat Massie, and I got somebody with a warm body, but a big, beautiful brain, and a great patriot."The other side: "They want 100% compliance," Massie said of the White House."I vote with the President 90% of the time. I voted for the SAVE Act. I voted for DHS. In fact, by most scorecards, I'm the most conservative Republican, so it's only the 10% of the time they're mad about."Between the lines: Trump has taken out a number of Republican politicians who cross him.Sen. Bill Cassidy (R-La.), a top target of the president, lost his reelection bid when he failed to finish in the top two in a primary Saturday.Trump also exacted retribution on a group of Indiana Republican state legislators who blocked his push to redraw the state's congressional map. Five of the seven lost their primaries last month.And Rep. Lauren Boebert (R-Colo.), who spent the weekend campaigning with Massie in his district, has now caught the ire of Trump. The president invited a primary challenger to run against her in a post on Truth Social Saturday, calling Boebert "weak minded."Catch up quick: Massie and Trump's contentious relationship dates back to Trump's first term.In 2020, Massie opposed Trump's Covid relief package, leading the president to call for Massie to be thrown "out of the Republican Party."Massie was also one of only two House Republicans to vote against Trump's "big, beautiful bill."Last summer, Massie sought to repair his relationship with the president, and Speaker Mike Johnson was able to broker a truce, Axios previously reported.The ceasefire didn't last long. Days later, Massie ramped up his public criticism of the administration's handling of the Epstein files, and later led the push to release the files in defiance of Trump.Zoom in: The race has spiraled into an especially nasty and personal fight, with both sides trying to portray the other as out of step with the GOP base on a range of culture-war issues.Outside groups aligned with Trump and pro-Israel donors have spent millions attacking Massie over his criticism of Israel and opposition to some foreign aid packages.Pro-Massie groups, meanwhile, have labeled Gallrein "Woke Eddie" and aired an AI-generated ad depicting the retired Navy SEAL abandoning Trump on a battlefield.One pro-Massie ad attacked Gallrein's ties to GOP megadonor Paul Singer — who is Jewish and supports LGBTQ rights — by featuring a rainbow-colored Star of David and warning that the "LGBTQ mafia" was trying to take over the district. "If Gallrein wins, the weirdos take over," the ad says.The bottom line: A loss for Massie would send another warning to Republicans about the dangers of crossing Trump — and show that the president's broader political problems haven't diminished his power with the base."If I lose, I think it's going to disenfranchise a large part of the coalition that was formed to give us the majority here and to give us the White House," Massie told reporters at the Capitol last week.