At the top of that list is someone once called the “accidental congressman,” Rep. Kerry Bentivolio (R-Mich.). As the first challenger, he won the seat in 2012 after the Republican incumbent failed to qualify for the primaries and subsequently resigned. Two years later, Bentivolio – a novice politician with no real chance of winning under normal circumstances – lost his primary by 33 points.
Representative Chris Bell (D-Tex.) lost a primary in 2004 by a 35-point margin, but that came after his district was massively overhauled, severely diluting white voters and opening the door to a black primary challenger.
Like these examples, most of the largest margins historically arose under unusual circumstances: dramatic reshuffles, party changes, scandals, or unusual primary trials. Many incumbents have lost double-digit primaries, and several have lost by 20 points or more, but mostly when these factors were present.
About the only party reprimand this century comparable to Cheney’s — both for the absence of those factors and the magnitude of the defeat — came in South Carolina in 2010, when Rep. Bob Inglis (RS.C.) was overcome by the tea party wave. But it took two candidates before it came close to losing Cheney.
Aside from the aforementioned races, the next biggest primary defeat may sound familiar: Rep. Tom Rice (RS.C.) earlier this year. Rice, like Cheney, voted to impeach Trump.