An expert witness in an ongoing federal trial on Wisconsin’s voter ID law testified on Friday that voter fraud is uncommon in Wisconsin, and that voter ID laws tend to be politically motivated.

"The Republican Party believes that, for their candidates to win, it's better to have more restrictive voting laws, and the Democratic Party believes the opposite," said Lori Minnite, a voter fraud expert and professor at Rutgers University in New Jersey. "You’ve seen this really stark partisan divide in every legislature that’s adopted voter ID laws in recent years."

Fraud prevention has been a primary argument of proponents of the law, which Gov. Scott Walker signed in 2011.

According to a study by Arizona State University cited earlier in the trial, there have been 57 allegations of voter fraud in Wisconsin between 2000 and 2013.

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Minnite was one of a few witnesses to take the stand during Friday’s court proceedings as the nine-day trial continued. The trial, resulting from a case brought by liberal group One Wisconsin Institute, is examining whether the voter ID law and more than a dozen other election-related laws have a disproportionate impact on minorities and young people.

In its defense, state officials countered that there's widespread public support for voter ID laws, and that the existence of fraud could influence the outcome of a close election.

Minnite agreed with both points.