Backpage.com is suing the sheriff in Chicago over his successful campaign to urge credit card companies to cut ties with the classified ads company over accusations that its sex ads facilitate human trafficking.

Backpage filed a federal lawsuit Tuesday alleging that Cook County’s Thomas Dart, sheriff of the second largest county in the country, was censoring the company and its customers’ right to free speech by pushing for Visa and Mastercard to end their relationships with Backpage. The two credit card companies announced earlier this summer they would stop processing payments to the company due to the concerns raised by letters Dart sent their CEOs.

“Sheriff Dart’s actions to cripple Backpage.com and all speech through the site are an especially pernicious form of prior restraint,” the complaint says. “He has achieved his purpose through false accusations, innuendo, and coercion, whereas, if he had brought suit directly or Cook County had attempted to pass a law to shut down the website, Backpage.com would have had a fair opportunity to respond and defeat such efforts, given well-established law.”

The suit was first reported by the Wall Street Journal.

“Sheriff Dart’s actions have not only infringed Backpage.com’s rights to publish and distribute speech, but the rights of millions of the website’s users to post and receive protected speech,” the complaint says.

For years, Backpage has been the target of anti-human trafficking advocates who say that the company has not done enough to crack down on the solicitation of sex services involving victims of forced prostitution, particularly underage victims. Backpage has said that it does work with law enforcement to curb trafficking and that ending its adult pages entirely will push forced prostitution to darker parts of the web, where illegal activity is harder to monitor.

The letters Dart sent to Visa and Mastercard last month said his office has made hundreds arrests in cases stemming from ads on Backpage.

“The use of credit cards in this violent industry implies an undeserved credibility and sense of normalcy to such illicit transactions and only serves to increase demand,” the letters said.

American Express had already stopped processing payments for Backpage’s adult services prior to Dart’s most recent complain.

An earlier TPM story about the credit card companies’ decision to drop Backpage is referenced in Tuesday’s complaint.

Backpage did not respond to request for comment, but Liz McDougall, general council to Backpage.com LLC, told the Wall Street Journal, “Our goal is to ensure that one elected official, particularly a county sheriff, cannot dictate what speech is or is not appropriate.”

The sheriff’s office fired back at Backpage after news of the suit broke.

“For years, Sheriff Dart has laid out to Backpage the numerous instances where pimps and traffickers have used their site for criminal purposes and attempted to negotiate in good faith with Backpage’s management to find common ground and put traffickers behind bars,” Dart’s spokesman said in a statement. “Unfortunately, this outreach was met with little more than delaying tactics and empty promises. Sheriff Dart requested that the credit card companies voluntarily do what Backpage will not – disassociate their business from online sex trafficking in the name of good corporate citizenship.”

Read the full complaint below: