As one of many Republican congressmen who just packed up their offices to make way for the Democratic majority, I’ve had time to reflect on what went wrong for the party. There are seismic political shifts under way on issues ranging from climate change to immigration to gun violence. If Republicans don’t adapt by offering meaningful solutions, the midterm losses could be only the beginning.

The disconnect may be widest on climate change, in which the gap between Republican voters and their elected leaders continues to grow....