Conor Lamb, a Democrat, pulled off a narrow but major upset by winning a special House election in the heart of Pennsylvania Trump country. Mr. Lamb won in the state’s 18th Congressional District, a reliably Republican seat in recent elections and an area that Donald J. Trump won by nearly 20 percentage points in 2016. The victory is an ominous sign for Republicans ahead of this year’s midterm elections.

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The results may still be contested, but Mr. Lamb’s current lead appears insurmountable based on the number of provisional, military and other absentee ballots left to count.

Now Mr. Lamb will have to decide soon which district to run in this year. The State Supreme Court threw out Pennsylvania’s current congressional map and recently issued a new map with redrawn boundaries. Tim Murphy, a Republican, resigned from the seat last year after reports that he encouraged a woman, with whom he had an affair, to have an abortion.

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