In the 2008 presidential election, 678,000 people voted early, a record number at the time.
Harris County Clerk Stan Stanart says even more are using early voting this year, with an estimated 700,000 casting ballots by the end of today.
"We've been pushing people to vote early because of redistricting. The fact that about 20 percent of the locations people normally vote at on election day could be different. Just to avoid that confusion, we've been asking people to vote early."
On election day, there will be 776 polling locations in the county with more than 10,000 e-slate machines to keep lines moving.
Stanart says there's still time to vote today and you don't need to check which precinct you're in.
"If you're in line at 7 p.m., you can vote tonight. Even though it might be 8 o'clock before you cast you ballot, if you're in line at 7, you get to vote today."
Stanart says early voting ballots aren't tabulated until shortly before 7 p.m. on Election Day.
For more information about voting locations and sample ballots, visit kuhf.org/election2012.