Illinois will hold its primary elections on Tuesday, with eyes on one congressional incumbent facing a challenge from within his party — joining several 2024 contests that have highlighted the fracturing of the Republican Party.

Rep. Mike Bost (R-IL), who represents Illinois’s 12th Congressional District, is facing a challenger from the party’s hard-line flank. Though Bost has the incumbent and fundraising advantages, as well as an endorsement from former President Donald Trump, he is facing competition from his own House colleagues as they throw support behind his challenger, Darren Bailey.

Bost’s Illinois primary is considered one of the tightest of the GOP’s 2024 races and the one that has drawn the most attention from the Land of Lincoln. Bailey, a right-flank candidate, lost the 2022 gubernatorial race to Gov. J.B. Pritzker (D-IL) and has been knocked by Republicans, including former House Speaker Kevin McCarthy, for a loss he claims cost the GOP four congressional seats in the midterm elections.

Bailey was one of several Trump-endorsed candidates who lost their elections in 2022 to Democrats after beating out centrist GOP primary challengers, which in part contributed to down-ballot losses and a slim House Republican majority.

Bost won the former president’s support for Congress after he vocally opposed President Joe Biden’s 2020 victory and also won the backing of Trump allies, such as Reps. Jim Jordan (R-OH) and Ronny Jackson (R-TX).

Firebrand Rep. Matt Gaetz (R-FL) and House Freedom Caucus member Mary Miller (R-IL) are throwing their support behind Bailey — a growing trend of House GOP members campaigning against their colleagues as they seek to bring new blood into the conference.

Gaetz and Bost have a complicated history, particularly after they sat on opposite sides of the House’s chaotic speakership vacancy and elections. Bost famously screamed at the Florida congressman when he was speaking on the House floor in January 2023 as he and other hard-line conservatives were withholding support for McCarthy’s speakership.

Bost also reportedly lunged at Gaetz during a closed-door hearing last October following the ousting of McCarthy by the Florida Republican and seven other GOP lawmakers.

The bad blood has spilled over into campaign season. In late February, Gaetz headlined a rally with Bailey in Herrin, Illinois. The Florida congressman has extended his fight against fellow House lawmakers to other states, as well.

He skipped the annual GOP retreat this past week and campaigned for Brandon Herrera, runoff primary challenger to Rep. Tony Gonzales (R-TX) — a centrist Republican who lost the support of national and state conservatives when he voted in support of bipartisan gun control legislation following the school shooting in Uvalde, Texas, which his district encompasses.

“House Republicans have been in a state of constant retreat. Congressman Gaetz is on the advance,” Gaetz’s spokesperson said of the congressman’s decision to not attend the GOP event in West Virginia.

Bost is not the only GOP member to face pushback from their House colleagues. Rep. Bob Good (R-VA), the chairman of the House Freedom Caucus, is in danger of losing his seat after he initially endorsed Gov. Ron DeSantis (R-FL) over Trump — later flipping to back Trump after DeSantis dropped out. Rep. Marjorie Taylor Greene (R-GA), a staunch ally of the former president, has endorsed Good’s primary challenger John McGuire, a former Navy SEAL and Trump supporter.

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Also at stake for Illinois’s Tuesday primary are 147 delegates for the Democratic presidential nominee, likely to go to Biden, and 64 delegates up for grabs for Trump, the all-but-guaranteed Republican presidential nominee. Both Biden and Trump reached the delegate threshold following the primary elections in Georgia, Washington, and Mississippi.

Other incumbents facing difficult primaries in Illinois include Rep. Chuy Garcia (D-IL), who is facing a challenge from Alderman Raymond Lopez, and Rep. Danny Davis (D-IL), who is up against a crowded field that includes the Chicago city treasurer and community members.