While many Americans were focused on Washington this week, I was paying special attention to Fort Bend County, Texas. What took place in that Houston suburb may reveal more about the 2020 election than the impeachment trial in the Senate does.

Fort Bend held a special runoff election to fill a vacant state House seat left open by the resignation of the Republican incumbent, who took a job with the University of Texas. Sensing an opening, state and national Democrats decided a win in House District 28 would give them a head...