Now Trump questions integrity of early voting with claims of 'vote flipping' on Texas touch-screens 

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Donald Trump ratcheted up his warnings about election fraud on Thursday, tweeting about 'vote flipping' on touch-screen voting machines in Texas.

'A lot of call-ins about vote flipping at the voting booths in Texas,' the Republican presidential nominee wrote on Twitter. 'People are not happy. BIG lines. What is going on?'

There are at least two reports of from the Lone Star State of votes being changed from Trump to Hillary Clinton when ballots are finalized on the machines.

Trump has warned for months about a 'rigged' election that could deny him a victory, and this week he chided President Barack Obama for criticizing that claim.

But Thursday morning's tweet marks the first time he has tied his predictions of voter fraud to a specific report that suggests he could be right.

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'A lot of call-ins about vote flipping at the voting booths in Texas,' the Republican presidential nominee wrote on Twitter. 'People are not happy. BIG lines. What is going on?'

There are at least two reports of from the Lone Star State of votes being changed from Trump to Hillary Clinton when ballots are finalized on the machines

On Monday a Texas woman named Shandy Clark posted a Facebook entry that became the talk of conservative radio shows, describing a family member's early voting experience in the city of Arlington.

'[S]he voted straight Republican. She checked before she submitted and the vote had changed to Clinton! She reported it and made sure her vote was changed back. They commented that it had been happening,' Clark wrote.

In an interview that aired Thursday morning on ABC's 'Good Morning America,' Trump seemed to soften his claim about a rigged election, saying more generally that 'I think the system is rigged.'

But that interview was taped on Wednesday. Eighteen hours later, the social media-driven candidate appears to have doubled down on his warnings about election fraud.

Shandy Clark posted a Facebook message describing a family member's early voting experience in the city of Arlington

Touch-screen balloting machines are susceptible to errors, but election officials typically blame voters for 'user errors.'

Tarrant County, Texas Elections Administrator Frank Phillips told Fox 4 in Dallas that 'we always call the sites and speak with the election judge to see if they have been told about this issue. 

'If the judge can confirm that someone has brought this issue up, we then take that machine out of service until we can test it.'

But 'we have never been able to replicate the issue, either at the site or in our office,' he claimed.

The problem has not been limited to Texas. In 2014 a YouTube video showed, on camera, a Virginia touch-screen voting machine recording a vote for a Democratic congressional challenger several times when a voter touched the check-box representing the Republican incumbent.

In 2014 a YouTube video showed, on camera, a Virginia touch-screen voting machine recording a vote for a Democratic congressional challenger several times when a voter touched the check-box representing the Republican incumbent

Texas officials say such 'flipping' problems can occur when voters incorrectly indicate that they intend to vote a 'straight party' ticket – and choose the wrong party

Texas officials, however, say such problems can occur when voters incorrectly indicate that they intend to vote a 'straight party' ticket – and choose the wrong party.

In a case like that, when a voter inadvertently chooses the Democratic ticket instead of Republican, the software would reject attempts to override that choice by picking individual Republicans.

But one voter told the Fort Worth Star-Telegram that a voting machine at a Dallas County library recorded her straight-ticket Republican vote and was surprised to see that it had recorded her as a straight-ticket Democrat.

In that case, she was able to avoid submitting the ballot when she saw the error on a summary screen. She said a poll worker apologized and told her to use another machine.

'My question is how many other people didn't know,' she told the newspaper.

The problem could actually be helping Trump in some cases, a wrinkle that didn't make it into his Thursday morning tweet.

Texas Democratic Party chairwoman Deborah Peoples told CBS 11 in Dallas that she has heard of 'multiple examples' of vote-flipping that takes votes away from Clinton.

'They voted for Secretary Clinton, it jumped to Trump,' Peoples said of the complaints she has heard, 'so we're working with the elections office.'