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The record was previously set during the 2012 November election and is expected to be near the one million voter mark, Harris County Clerk Stan Stanart said.
Another 170,000 to 190,000 votes are expected to be cast Thursday and Friday.
"Incredible turnout for a historic election," said Stanart in a news release. "I expect the total number of early voters for this election, including mail ballots received, will get close to the one million mark by the end of the early voting period."
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The early voting period ends Friday.