April 17, 2018
Rep. Charlie Dent announced he would resign from Congress "in the coming weeks". The Pennsylvania Republican, in his 7th term, had previously announced he would not seek re-election in 2018.
Pennsylvania Gov. Tom Wolf must announce a special election date within 10 days of a vacancy. Complicating matters is this year's court ordered redistricting in the state. Dent's 15th congressional district will largely become part of the new 7th district. However, any special election would take place using the current district boundaries, as these remain effective for purposes of representation until the new Congress is seated in 2019.
There may be a way around this - we don't know - but here's an interesting scenario:
Given the timing, from a cost perspective it likely makes the most sense to have this special election on the same date as the midterm elections. When these concurrent elections happen, the same nominees are usually on the ballot for both races, and thus the winner of the special election is normally also going to be the winner of the election for the subsequent full term.
However, in this case, two different sets of voters would be involved on the same day - those in the current 15th district for the special election and those in the new 7th district for the full term. While the 15th district voted for Trump by about 8% in 2016, the new 7th district actually voted for Clinton by 1%. As a result, it is possible that the winner of the special election could lose the regular election and thus this person would only serve in Congress during the few weeks of a lame-duck session.