Bell unseats Bush. How often does a primary challenger defeat a Congress incumbent?

ST. LOUIS - In one of the most expensive and anticipated primary races for U.S. Congress this year, challenger Wesley Bell defeats incumbent Cori Bush for Democratic nomination of Missouri Congressional District 1, a seat representing the St. Louis-area.

Bell won nomination to run as the Democratic candidate in the November 2024 general election. Bush has held the seat for the last four years, and her current term will end in January.

By winning the primary as a challenger, Bell accomplished something that isn't attained often: A challenger defeating an incumbent for a U.S. Congress seat in a primary election. According to political consulting firm CN4 Partners, historically, only around 6% of Congressional incumbents have lost reelection bids in the U.S. primaries. 

A 2018 study released by Brookings Institution<...