Gavin Newsom is having a moment. In the national struggle over U.S. House control, perhaps no politician has more at stake than the California governor, who has emerged as the leading Democratic adversary to President Donald Trump in what many see as a lightly masked trial run for his own future White House bid.

Nearly half of California voters support Gov. Gavin Newsom (D)’s push to create more safe Democratic congressional seats before the midterm election, a new poll found, but support hasn’t crossed the c...