On Tuesday, all eyes will be on Virginia’s 5th Congressional District, where House Freedom Caucus Chairman Bob Good could become the second incumbent ousted in a primary this cycle if former President Donald Trump has his way.

“If he’s reelected, Bob Good will stab Virginia in the back, sort of like he did with me,” Mr. Trump, the presumptive GOP presidential nominee, said Monday evening on a 12-minute tele-rally for Mr. Good’s primary opponent, state Sen. John McGuire.

Mr. Trump holds a grudge against Mr. Good for endorsing Florida Gov. Ron DeSantis in the presidential primary and only endorsing Mr. Trump after the governor dropped out — which the former president dismissed Monday as “not good enough.”

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“John McGuire has my complete and total endorsement. I want that to be understood because Bob Good’s going around saying we’re friends,” Mr. Trump said. “I mean, he was fine the last six months, but before that, he was a basic disaster.”

The only incumbent to lose so far in 2024 primaries is Rep. Jerry Carl, but that wasn’t an analogous primary “challenge.” Fellow Rep. Barry Moore defeated him after a Supreme Court-ordered redistricting of Alabama’s congressional map put the two Republicans in the same district.

Mr. Good, who is still trying to run as pro-Trump, is confident he won’t become the first incumbent bested by an outsider this cycle.

“There are people with their own dishonest agenda, who don’t have the president’s interests in mind, who are in his ear lying to him about me and lying to him about this race,” Mr. Good said Saturday at a campaign stop in Campbell County.

“Who here thinks President Trump knows me better than you do?” he said.

Mr. McGuire, a former Navy SEAL, said on Monday’s tele-town hall that he’s “100% America First” and that electing him in the primary will help Mr. Trump win Virginia in the general election.

“We can lock arms and make it ’too big to rig’ so we win Virginia for President Trump in November, so we can get him back in there and make America great again,” he said.

The 5th District covers a large swath of central and southern Virginia from Charlottesville to the North Carolina border.

Mr. Good has represented the district for two terms since ousting former Denver Riggleman, also a Republican, in 2020.

But that victory, as well as his renomination in 2022, came at a party convention. Tuesday is the first time Mr. Good will have to rely on Republican voters across the district in a traditional primary contest.

Five publicly released polls of the race have split on who is favored to win.

Mr. Good has far more endorsements from local GOP officials than Mr. McGuire, but both men have lost support in their home counties. Mr. McGuire has faced criticism for breaking a promise made during his 2023 state Senate campaign that he would not run for Congress against Mr. Good.

Mr. Trump’s endorsement against Mr. Good heightened national attention on the race, which was already dividing congressional Republicans.

Mr. Good has support from most members of the House Freedom Caucus and other like-minded members like Rep. Matt Gaetz, who led the push, joined by Mr. Good, that ousted Speaker Kevin McCarthy of California. He also is endorsed by anti-establishment Republican Sens. Rand Paul of Kentucky, Mike Lee of Utah and Rick Scott of Florida.

Mr. McGuire is backed by several center-right House Republicans who dislike Mr. Good for voting to depose Mr. McCarthy and leading blockades of GOP legislation he deemed insufficiently conservative.

Rep. Marjorie Taylor Greene, an ally of Mr. Trump and Mr. McCarthy who was booted from the Freedom Caucus, has also endorsed Mr. McGuire and campaigned in the district with him.

Mr. McGuire also won a late endorsement from one of Mr. Good’s Freedom Caucus colleagues, Rep. Warren Davidson.

The Ohio Republican is no fan of Mr. Good or his hard-line tactics, having resigned from the Freedom Caucus executive board in protest to him being named chair.

“John McGuire is conservative and effective,” Mr. Davidson said. “He will work well with others to deliver conservative results.”

Mr. McGuire had raised $1.24 million and spent a little over half of it, while Mr. Good raised $1.15 million and spent most of it as of May 29, according to Federal Election Commission records.

Outside groups supporting both candidates have collectively poured more than $10 million into the race, according to FEC filings compiled by ProPublica.

Mr. Good’s campaign announced Monday that it had made 208,000 direct voter contacts, touting that as more than five times the Cardinal News-projected voter turnout for the primary.

The Good campaign also sent a message to constituents Monday urging them “to send a resounding message to the DC Swamp” by voting for Mr. Good and “Christian Conservative principles.” The message made clear that if Mr. Good wins, he won’t harbor any resentment toward Mr. Trump.

“Following the election, we will harness our collective dedication, expertise, and resources to secure a victory for President Trump in Virginia this November,” the campaign wrote. “Together, we will Make America Great Again.”