Harris County added more than 11,000 voters to its rolls in the final week before the registration deadline, the last wave in a surge of half a million new Texas voters since the March primaries.

Democrats are most likely to benefit from the increase because new voters, many of whom are young and/or nonwhite, are more likely to support their party, University of Houston political science Professor Brandon Rottinghaus said.

“There is a long legacy of Democrats seeking to get more people registered, and the investment is likely to pay off,” Rottinghaus said. “This is a moment where there’s going to be a lot of nail biting from Republicans on election night.”

More than 66,000 residents registered to vote in Harris County since the spring, more than any other Texas county, according to the Texas Secretary of State. Since the 2014 midterms, Harris County has added 280,000 voters.

Harris County Tax Assessor-Collector Ann Harris Bennett, who also serves as the county voter registrar, did not respond to a request for comment. Her office said Tuesday it had yet to calculate how many new voters had registered since September.

More Information Ten Counties with the biggest percentage increase in voter registration since the Texas Primary elections 1. HAYS - 8 percent 2. BRAZOS 5 percent 3. COMAL - 5 percent 4. TRAVIS -5 percent 5. RAINS - 5 percent 6. GUADALUPE - 5 percent 7. NACOGDOCHES - 5 percent 8. WILLIAMSON - 5 percent 9. KING - 5 percent 10. COLLIN - 5 percent

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Rottinghaus cautioned that there is a poor correlation between voter registration and turnout. Even as more eligible Harris County voters have registered since the 1990s, turnout has declined. Republicans, he said, are hampered by their past success since they already have registered most of their potential voters. Democrats have more room to grow, he said, especially with Latinos, African Americans, new citizens and young people.

Harris County’s largest political parties had different takeaways from the voter registration surge.

Democratic Party spokesman Odus Evbagharu said the jump in new voters will help the party dominate Harris County elections as it did in 2016, even as he acknowledged the challenge now lies with convincing those voters to actually show up at the polls.

“Whether by phone, text or social media, we are contacting them constantly to remind them there is an election,” he said.

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Vlad Davidiuk, communications director for the Harris County Republicans, attributed the surge to registration drives by both parties.

“Since both parties have been working overtime to get their voters registered, and their registrations updated … it does not indicate a surge of Democratic voters,” he said.

New numbers from the Texas Secretary of State show a record 15.7 million voters will be eligible to vote in the Nov. 6 midterm elections, which feature one of the hottest U.S. Senate races in the country between incumbent Republican Ted Cruz and Democratic Congressman Beto O’Rourke, the governor's race, and more than a half dozen highly competitive congressional races.

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The two-week statewide surge is more in line with the number of voters Texas typically adds in an entire year. From 2002 to 2014, Texas added 116,000 voters a year.

And the total is likely to rise as last-minute voter registrations are reported to the Texas Secretary of State’s office. The last day to register to vote in time for the midterm elections was Tuesday. The numbers come even as more than 2,000 voters had their registrations deemed incomplete because they registered online via the national Vote.org. Those voters have 10 days to resubmit their registrations.

For subscribers: Houston Chronicle’s Voter Guide

For subscribers: San Antonio Express-News Voter Guide

Across Texas, voter registration increased 2 percent since March primary elections.

In that time frame, no county has seen a jump in voter registrations larger than Hays County, just south of Austin and home to San Marcos, one of the fastest-growing metro areas in the United States. Hays has reported an 8 percent increase in voter registration since the March primary elections.

Major metro counties, like Dallas, Bexar, and Harris counties have seen their voter registrations increase 3 percent since March.

Suburban counties have seen a bigger jump. In Fort Bend and Montgomery counties near Houston, voter registrations grew 4 percent since March. In Collin County near Dallas, voter registrations climbed 5 percent.

In Guadalupe and Comal counties north of San Antonio, voter registrations jumped 5 percent.

Texas Secretary of State Rolando Pablos has said the surge of voter registrations and enthusiasm in this midterm election cycle is akin to what election officials often see in presidential election cycles.

Early voting starts on Oct. 22, and Election Day is Nov. 6.

zach.despart@chron.com

jeremy.wallace@chron.com