Despite repeated statements by Republican political leaders that American elections are rife with illegal voting, credible reports of fraud have been hard to find and convictions rarer still.

That may help explain the unusually heavy penalty imposed on Rosa Maria Ortega, 37, a permanent resident and a mother of four who lives outside Dallas. On Thursday, a Fort Worth judge sentenced her to eight years in prison — and almost certainly deportation later — after she voted illegally in elections in 2012 and 2014.

The sentence for Ms. Ortega, who was brought to this country by her mother as an infant, “shows how serious Texas is about keeping its elections secure,” Ken Paxton, the Texas attorney general, said in a statement. Her lawyer called it an egregious overreaction, made to score political points, against someone who wrongly believed she was eligible to vote.

There is, however, at least one flaw in that story: Ms. Ortega was a registered Republican.

“She voted for Mitt Romney over Barack Obama in the 2012 election. In 2014 she voted for our current attorney general, Ken Paxton,” Mr. Birdsall said. “And guess what? He’s the one responsible for prosecuting her.”