To regain Senate control, Republicans must defeat at least three incumbents, something they haven’t done since 1980 when they defeated 12. The most incumbents Republicans have knocked off since then are two each in 1994 and 2010.

Voters will replace Democratic incumbents when they are convinced they are too closely allied with Mr. Obama, and when the Republican alternative represents constructive change. Republicans should resist the temptation to focus exclusively on the former while neglecting the latter.

Mr. Obama remains something of a wild card. His profile has receded this summer as international crises, private fundraisers and his vacation put him largely out-of-sight, out-of-mind. But he will put himself back at center stage this fall, believing that his involvement is essential to raise money and rally the Democratic base, especially blacks and young women.

Events will also put Mr. Obama front and center. Congress must soon approve a continuing resolution to fund the federal government. Republicans favor longer-term funding while Democrats want a shorter duration.