What if it's President Harris, but a GOP Senate? That's her allies' fear.

Should Kamala Harris win in November, her administration could begin in the weakest starting position of any in a generation. Her allies are already fretting over what to do about it.

Democrats close to the vice president have grown increasingly worried that Republicans will flip the Senate next month even if Harris wins — a scenario that would make Harris the first president since George H.W. Bush to start her term without a majority in the upper chamber.

The fears have sparked...