Democrats are banking on a rare chance to regain ground in Iowa, a rural state that has repeatedly eluded them in recent years. Republicans, meanwhile, are grappling with a New Jersey congressman whose unexplained absence could put their already slim majority at risk.

While Republicans hold an advantage in Iowa, Democrats see openings in an open governor’s and Senate race, and two congressional districts.

California heads toward an election Tuesday with its two marquee races defined by uncertainty, while a pair of outsider candidates looked to crack open the state’s durable…