The number of politically competitive U.S. House seats is shrinking rapidly. There are many reasons. People have stopped splitting their tickets when voting and now usually support one party in all races. More people now live in areas with similarly-minded voters. Swing seats have taken a hit since President Donald Trump kicked off an unusual round of mid-decade redistricting last year. The impact is clear in one North Carolina district that went from competitive to safely Republican. The loss of swing districts adds to polarization because most congressional races are decided in party primaries.