Rep. Tim Holden (D-Pa.) lost his primary on Tuesday in the face of a strong challenge from the left.
Holden was challenged by attorney Matt Cartwright (D), a self-funding candidate who spent freely on television ads.
With 80 percent of precincts reporting, Cartwright had nearly 59 percent to Holden's 41 percent.
Holden, a Blue Dog Democrat, has long represented a Republican-leaning district based in Harrisburg, winning most elections without difficulty and maintaining a strictly centrist voting record. But his district got significantly more Democratic in the redistricting process.
In the primary, he had to contend with the Campaign for Primary Accountability, a super-PAC that targets vulnerable long-term incumbents of both parties. The group has spent more than $200,000 on the race.
Holden is the fourth incumbent to lose in a primary this cycle, following Reps. Jean Schmidt (R-Ohio), Dennis Kucinich (D-Ohio), and Dan Manzullo (R-Ill.).
Schmidt was also a victim to the Campaign for Primary Accountability while Kucinich and Manzullo lost in primaries to fellow lawmakers.
In a statement on the primary, the super-PAC said: "The Campaign for Primary Accountability achieved its goal of helping voters hold elected officials accountable through competitive elections. "