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 10-term incumbent, is facing a formidable challenge from Matt Cartwright, an attorney who is running as the more liberal candidate in the race. He's also better known in Scranton, Pa., a town drawn into Holden's district after the once-a-decade reapportionment process cost Pennsylvania a congressional seat.

But the race may be most significant as a demonstration of just how big a role outside spenders are poised to play in congressional contests: the independent expenditures in the district equal 44 percent of what the two candidates reported spending combined in their most recent reports to the Federal Election Commission: Through April 4,  Holden had reported expenditures of $855,000 and Cartwright, $427,000.

Complicating Holden's prospects: the lawmaker, one of the most conservative Democrats in Pennsylvania's congressional delegation, according to rankings by the non-partisan National Journal, is caught in a cross-fire between liberal and conservative outside interest groups that are working for his defeat. Last Congress, he helped kill his party's so-called cap-and-trade legislation, which aimed to reduce greenhouse gas emissions by putting a price on the use of carbon-based fuels, saying it would hurt coal mining interests in his district. 

The League of Conservation Voters has spent $238,000 on the race, most of it for a TV ad that suggests Holden's policies on energy and the environment are closer to those of former President George W. Bush than they are to President Obama's. The Campaign for Primary Accountabiity, an anti-incumbent super PAC that claims non-partisanship but has links to the Tea Party movement, has also gotten into the act, buying ad time to tout Cartwright as the "true-blue Democrat" in the race. 

Holden, meanwhile, is getting help from Center Forward, a group founded by one of his former congressional colleagues from the Blue Dog Democrat coalition of fiscal conservatives. Center Forward has reported spending $104,000 on TV ads opposing Cartwright.