There are four contests in states Mr. Trump carried in 2016 and should carry again. But Arizona’s Martha McSally, Iowa’s Joni Ernst, Montana’s Steve Daines and Georgia’s David Perdue must campaign hard and effectively. It’s hardly a foregone conclusion Mr. Trump will win all their states.

Also, while there are advantages to Minority Leader Chuck Schumer’s practice of often recruiting candidates with no prior political service, the drawback is that these Democrats haven’t been vetted. Mark Kelly in Arizona turns out to have recruited a major Chinese investor for his space-tech company that’s described as “an arm of the Chinese Communist Party.” Iowa real-estate executive Theresa Greenfield has a record of harsh evictions. Nine of the 10 documentaries Georgian Jon Ossoff’s production company made were for Al Jazeera. If Republicans carry three or four of these races, they’ll be knocking on the majority.

The GOP’s three toughest contests are re-electing Cory Gardner in Colorado, Susan Collins in Maine and Thom Tillis in North Carolina. Sens. Gardner and Collins are their party’s two best candidates this cycle, running energetic, nearly textbook-perfect campaigns. Both are capable of being in that 12% who win a state their party loses in the presidential race. Mr. Tillis, an outstanding legislator, is neck-and-neck in a state where Mr. Trump has a slight advantage. Win one or two and the GOP could well be in the majority.