Voters casting their ballots early in Texas claimed this week that when casting Democratic or Republican straight-ticket ballots, voting machines in at least 80 counties in the Lone Star State flipped their votes to the other party in key midterm races, including the high-profile competition between incumbent Sen. Ted Cruz and his Democratic opponent Beto O'Rourke.

The Texas secretary of state's office on Friday said that there have been issues reported with Hart eSlate voting machines, which are used in approximately 30 percent of counties statewide. The machines feature a wheel for selecting candidates and buttons to move from screen to screen.

But the secretary of state's office said the issues were not caused by the voting machines, but by voters themselves.

"The Hart eSlate machines are not malfunctioning, the problems being reported are a result of user error — usually voters hitting a button or using the selection wheel before the screen is finished rendering," said Sam Taylor, spokesman for the office of Secretary of State Rolando Pablos.

The Houston Chronicle reported that Democratic voters claimed the machines indicated that they were about to cast a vote for Cruz, when they originally intended to choose O'Rourke. Several people said they were able to get help from employees at polling places in order to change their vote back.

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Mickey Blake, a Houston resident, told KTRK-TV that she voted for Democrats down the ticket but saw before she submitted it that O’Rourke’s name was not selected.

The machines are used in around 80 counties in the state. Early voting in Texas began Monday and has featured a strong turnout.

The machine’s manufacturer, Hart InterVCivic, attributed the issues to 16-year-old technology and defended the machines' accuracy.

"The same story has happened in multiple elections," Steven Sockwell, the company's vice president of marketing, said Friday. "There was no flipping then and there's not any now."

In a statement to supporters, Cruz cited "multiple reports" of vote changing and added, "Once you select the Republican party ticket, please be patient and do not select 'next' until the ballot has populated all of the selections."

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The Texas Democratic Party called the issue "a malfunction," saying it was causing Democrats to inadvertently vote for Cruz and accused the secretary of state's office of not doing enough to warn voters of potential issues.

Taylor said his office “has no legal authority whatsoever to force any” voting machine vendors “to make upgrades if their voting systems are otherwise in compliance with federal state and law.” He said the Hart eSlate’s system was certified in 2009. He also said it was the county’s responsibility to purchase new voting equipment if needed.

"We will continue to educate Texas voters using existing resources," Taylor said, "and urge all Texans casting a ballot to take their time, slow down, and carefully review their ballot before casting one."

The Houston Chronicle noted this will be the last election voters can vote for a straight ticket.

The Associated Press contributed to this report.