Reliably Democratic Illinois is nobody’s idea of a swing state.

But three heated House primaries in the Land of Lincoln next week illustrate the broader vulnerabilities of both major political parties going into the general election: age, extremism and immigration. In today’s newsletter, I’m going to tell you about some fascinating primary races that will shed light on some broader trends in U.S. politics.

Let’s start with Illinois’s 12th Congressional District, in the southern part of the state. Mike Bost, a Republican and Marine Corps veteran, was first elected to the House in 2014. Democrats tried to tar him as “Meltdown Mike,” highlighting his angry outbursts in the State Legislature and warning, “He’d make Washington worse.”

Well, those were simpler times. A decade later, Bost is what passes for an establishment Republican. He is the chairman of the House Committee on Veterans’ Affairs and sits on the committees on agriculture and transportation, from which he can steer money and projects to the largely rural district that stretches across the bottom third of the state.

His primary opponent, Darren Bailey, is proving that in the era of Donald J. Trump, there may be no limits to the G.O.P.’s rightward drift. Bailey, as you might recall, was the ardent, pro-Trump Republican whom Illinois’s Democratic governor, J.B. Pritzker, spent big money to elevate in the Republican primary for governor in 2022, figuring he’d be easy to beat — which he was. Pritzker won by nearly 13 percentage points.

Bailey is calling Bost “Amnesty Mike,” an insufficient apostle of Trump’s “America First” agenda. But Bost has Trump’s endorsement. And to make matters even more interesting, Bailey has been endorsed by Matt Gaetz, a high-profile Trump ally and firebrand, who has had heated run-ins with Bost. It’s all enough to spin heads.

Don’t say ‘age’

Democrats have their own issues that are captured in races in their stronghold of greater Chicago. Let’s start with age: Danny Davis has represented a swath of Chicagoland stretching from Lake Michigan to the western suburbs for nearly 28 years, and at 82, he’s determined to stay in Washington.