PA-15: Rep. Charlie Dent (R) to retire

[Attorney General & PPT Dwarven Dragon]

Oof! He was the GOP's only hope of ever winning another senate seat in PA.

That's a bit drastic. Barletta v. Casey '18 is only Lean D.

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This seat may be something democrats look at. Dent was very popular and won by landslides as a result, but at the top of the ballot this is a republican-leaning swing seat:

2008: 52-47 Obama
2012: 51-48 Romney
2016: 52-44 Trump

[Attorney General & PPT Dwarven Dragon]

Oof! He was the GOP's only hope of ever winning another senate seat in PA.

If he could even win the primary (I doubt he would), I bet a lot of conservatives, especially those who are pro-lifers and/or Trumpists, would either stay home, vote third party, or even defect to Casey. This isn't Hotelling's model, where you race to the center to win, winning these days is really about firing up the base (and keeping them on your side).

Indeed! That's why Romney did so well when he appealed to the base on immigration, taxes, guns, and gay marriage.

Romney never attacked Obama on gay marriage. (GOP-affiliated groups did, but not Romney himself) And I don't recall guns being a huge issue in the '12 campaign.