Tuesday's battle for the Democratic nomination in Pennsylvania's newly-redrawn 17th Congressional District has emerged as the biggest money magnet so far for outside groups spending on House primaries, data compiled by the Sunlight Foundation's Follow the Unlimited Money tracker shows. 

Outside groups have spent more than $572,000 in the semi-rural central Pennsylvania district, most of it to defeat  Rep. Tim Holden. The only other House races that have drawn more outside spending so far have been special elections in which Democrats and Republicans were competing.

By contrast, the Pennsylvania race is a Democrat-on-Democrat affair: Holden, a ...