• by Wendy Davis , Staff Writer @wendyndavis, October 27, 2015

The libertarian group Cato Institute is siding with Backpage.com in its ongoing battle with Cook County Sheriff Thomas Dart over adult ads.

The Cato Institute, along with other libertarian organizations, is urging the 7th Circuit Court of Appeals to ban Dart from continuing with his efforts to defund Backpage.com. Earlier this year, Dart complained about Backpage.com to Visa and Mastercard. Both companies later said they would no longer process credit card transactions for the site.

"Dart’s threat poses a grave threat to individual rights," the groups say in a friend-of-the-court brief filed this week.

They add that the sheriff's initiative could backfire by making it more difficult for law enforcement officials to combat crime, arguing that credit card records are a "valuable investigatory tool."

"Without the credit card records that Backpage.com provides to law enforcement as a matter of course... investigators will be hamstrung in preventing sex trafficking," the groups say.

"Were Sheriff Dart devoting his department’s resources to pursuing actual instances of human trafficking or fraud instead of threatening credit card companies, there would be a much better chance of actually reducing the number of trafficked persons and preventing child prostitution," the Cato Institute argues in its papers.

Dart has long criticized Web companies that allow people to post ads for "adult services," arguing that the vast majority of those listings are thinly disguised prostitution ads.

Years ago, Dart sued Craigslist for allegedly creating a public nuisance by allowing such ads. Craigslist won the lawsuit, but later decided to ban ads for adult services.

After Craigslist banned adult services ads in 2010, the listings largely migrated to other sites, including Backpage.com.

Earlier this year, when Dart reached out to Visa and Mastercard, he specifically asked the companies to stop processing payments for Backpage.com's adult services listings.

The classifieds site subsequently alleged in a lawsuit that Dart's complaints jeopardizes its business -- including its ability to offer users a forum for free speech. Backpage sought an injunction banning Dart from continuing to lobby against the company.

Dart countered that he is entitled to air his criticisms of Backpage.

U.S. District Court Judge John Tharp, Jr. in Illinois initially sided with Backpage and issued an injunction against Dart.

Several weeks later Tharp changed course and allowed the order to expire. He said that Mastercard and Visa were already considering ending their relationship with Backpage before Dart got involved. Backpage is now appealing that decision to the 7th Circuit.

The appellate court is expected to hold a hearing on Nov. 13.