Why did popular-vote winners still lose key Senate races?
Core argument: Republicans won the national U.S. House popular vote by three points, 51 percent to 48 percent, and still led by two points after adjusting for races in which only one major party appeared on the ballot.
Republicans won the national House popular vote by three percentage points — 51 percent to 48 percent. They still won by two points after adjusting for races in which only one major party was on the ballot. Republican candidates won the most votes for U.S. House in all four of the crucial Senate states where Republicans fell short: Pennsylvania, Arizona, Georgia, and Nevada. The “MAGA” Republicans — as characterized by The Cook Political Report, based on their backing from Mr. Trump in the primaries — ran far behind the mainstream Republicans.

