In the June primary, Caruso led Bass early on but then slipped into second place a week later. In the end, the congresswoman came out 7 points ahead.

A law that took effect last year requires county elections officials to send mail-in ballots to all active registered voters. A mail ballot can be postmarked until 8 p.m. on election day and arrive up to 7 days later and still be counted.

Caruso, a billionaire former Republican, has poured roughly $100 million of his own fortune in the race. Bass, who was elected in 2010 after serving as speaker of the state Assembly, ran with the support of nearly the entire Democratic Party establishment.