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.

The Associated Press projected Cartwright's win a little after 10 p.m.

With 80 percent of precincts reporting, Cartwright had nearly 59 percent to Holden's 41.

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 became 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 Don Manzullo (R-Ill.).

Schmidt also fell 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."

— This story was updated at 10:50 p.m.