Pennsylvania GOP Rep. Tim Murphy's resignation following revelations of text messages appearing to ask a mistress to have an abortion sets up new special election drama Republicans didn't want or anticipate. The 18th CD, in Pittsburgh's southern suburbs, is quite red and has comfortably reelected Murphy since 2002. However, special elections following scandals often produce surprises. It moves from Solid Republican to Likely Republican.

When Murphy indicated yesterday he would retire in 2018 rather than resign, we didn't move PA-18 because it was unclear the tarnished incumbent's exit would actually improve Democrats' odds. But a low-turnout special scenario in a pro-Democratic climate adds unpredictability.

Under Pennsylvania election law, Democratic Gov. Tom Wolf has ten days to call a special election to be held no sooner than 60 days from the date of his proclamation. That means a race to replace Murphy could be held as soon as early December or as late as the middle of 2018. Party committees will select nominees, and given Democrats' strong showings in similarly red districts earlier this year, both parties will have no choice but to take the race seriously.

Pennsylvania's 18th CD gave President Trump 58 percent of the vote last November and has a Cook PVI score of R+11 - making it slightly more Republican than GA-06 and SC-05, slightly less Republican than KS-04 and roughly on par with Montana. Of course, Democrats narrowly lost all of those races, but there are three reasons why a special election in PA-18 could hold more promise for Democrats than its recent statistics indicate.

First, despite its support for Trump, the southwestern corner of the state has a Democratic heritage: Democrats still hold a six point party registration advantage in the 18th CD, and plenty of local officeholders are still Democrats. In 2010, Democrats bucked the national pro-GOP mood and the area's pro-GOP trend and won a special election following the death of Rep. John Murtha in the 12th CD, which overlaps with much of the 18th CD's current territory.

Second, disturbing scandals like Murphy's can generate surprising backlash against the perpetrator's party in special elections - even if the replacement nominee is totally untarnished. Since 2010, only four of 35 House specials have produced a party change, and all four involved scandals. Two of the most shocking upsets came in 2011, when Democratic Rep. Anthony Weiner (NY-09) and GOP Rep. Chris Lee (NY-26) resigned after compromising selfies surfaced.

Third, as we've seen in other races this year, a special election is likely to attract a smaller electorate that favors the more motivated party. Today, that's still Democrats.

Before Murphy's resignation, GOP state Sen. Guy Reschenthaler had already announced for the seat. As a former Navy JAG in Iraq and the head of his party's state senate campaign arm, he may have the inside track. On the Democratic side, VoteVets-endorsed former assistant VA Secretary Pam Iovino is running along with former teachers' union official Mike Crossey and physician Bob Solomon, but the prospect of an open seat could attract others.

Bottom line: this is very tough territory for Democrats. But it's not impossible, and the combination of a pro-Democratic national climate and the fallout from Murphy's ignominious personal behavior makes this special election potentially competitive.