Voter turnout in the county is expected to be higher than usual with races for the 39th, 45th and 48th districts drawing national attention.
Workers at the Orange County Registrar's Office had already counted 400,000 ballots. The number of ballots that come in Tuesday could push results into later this week, possibly into next week for tight races.
"This more like a presidential election," said O.C. Registrar of Voters Neal Kelley. "When you look at the volume and the amount of people that are interested, it is definitely presidential scale.
"We plan about a year out," Kelley added, "and I knew there was going to be a lot of activity to this midterm but I'm surprised by this."
The office spent 11 days counting the 400,000 ballots of those who voted early. Kelly predicted that close to 750,000 Orange County voters might cast ballots in the election.
"For a midterm, that is historic," he said.
In 2014, the last midterm election, turnout in the county was 42 percent.
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