Pennsylvania GOP Rep. Charlie Dent's decision to retire in 2018 not only gives Democrats another valuable pickup opportunity, it further depletes the ranks of moderate Republicans in the House. In his statement, Dent cited "disruptive outside influences that profit from increased polarization and ideological rigidity that leads to dysfunction, disorder and chaos." Dent never endorsed Donald Trump in 2016 and voted against the GOP's healthcare repeal bill.

Dent had been flirting with danger from his right flank before his announcement. Although Dent had dodged a conservative primary challenge for years, state Rep. Justin Simmons had been gearing up to run in 2018, accusing Dent of "going off the rails" on President Trump. Now, the possibility that Republicans will nominate a more ardent conservative to succeed Dent suddenly makes this Lehigh Valley seat more intriguing for Democrats.

A decade ago, PA-15 was a classic swing district based in the Lehigh Valley and Dent fended off some well-funded Democratic challengers. But in 2012, GOP mapmakers pushed it west to include Republican parts of Lebanon and suburbs of Harrisburg, which could make it tougher for Democrats to win as an open seat. The district voted for Mitt Romney by three points in 2012 and for Trump by eight points in 2016.

Still, the potential for a Democratic wave election means this seat will be in the thick of the battle for control of the House. This is a place where county party organizations still matter, and a lot depends on whether Republicans nominate someone much more ideological than Dent. For now, we are moving PA-15 from Solid Republican to Lean Republican, but the right Democratic candidate (or the wrong GOP one) could make it even more competitive.