AUSTIN, Texas (AP) — Some Texas voters are complaining that machines flipped their straight-ticket selections to the other party in key races during early voting, especially the much-watched Senate battle between Republican Ted Cruz and Democrat Beto O'Rourke.

The secretary of state's office said Friday that there have been reported issues with Hart eSlate voting machines, which are used in around 30 percent of counties statewide and feature a wheel for selecting candidates and buttons to move from screen to screen. But it says they are caused by voters themselves and often occur when they complete and submit ballots too quickly.

"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, who was appointed by Republican Gov. Greg Abbott.

The machines are used in around 80 counties, including the state's largest, Harris, which is home to Houston, as well as Travis, which includes Austin, and Tarrant, encompassing Fort Worth. Early voting in Texas began Monday and has featured strong turnout and long lines. It runs through Nov. 2, ahead of Election Day on Nov. 6.

The Texas Hart eSlate machines do not provide receipts or other forms of paper trail to voters, but those casting ballots do see a screen that shows their choices before final submission — and can go back and make changes. Complaints about flipped votes have cropped up elsewhere in past election cycles. Similar machines are used in parts of Indiana, Kentucky, Pennsylvania, Tennessee and Virginia, according to Verified Voting, a nonprofit group focused on ensuring the accuracy of elections.

In a statement to supporters Friday, Cruz cited "multiple reports" of race selections 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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An advisory to county clerks and elections administrators issued Tuesday by Keith Ingram, the secretary of state's office's director of elections, said, "We have heard from a number of people voting on Hart eSlate machines that when they voted straight ticket, it appeared to them that the machine had changed one or more of their selections to a candidate from a different party."

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

Party chairman Gilberto Hinojosa said in a statement that "Texas' Republican government blamed voters and did nothing." He called for a statewide public service announcement to warn voters, training for poll workers on the issue and removal of "all malfunctioning machines."

Taylor said Friday that his office "has already trained election officials across the state at our annual election law seminars, instructed election administrators to post additional signage in multiple languages, required county officials to keep a detailed, meticulous log of any malfunctioning machines, and remove any machines that are malfunctioning."

Taylor also 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 and state law," and that Hart eSlate's system was certified in 2009. He said counties are responsible for purchasing their own new voting equipment.

"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."