When it comes to battling prostitution and child exploitation, the sheriff for the nation's second-largest county is walking tall and carrying a big stick.

That's why Illinois, Cook County Sheriff Thomas Dart and the operators of online classified portal Backpage.com are in a legal duel of sorts—and the First Amendment and federal law protecting website operators are squarely in the crosshairs.

The latest public battle between them commenced July 1, when Dart announced that Visa and MasterCard, at his urging, agreed to stop processing ad payments for what the sheriff described as "sex trafficking industry profiteer Backpage.com."

According to the sheriff:

Such ads – millions of them posted a year – make up the foundation of a booming modern sex trafficking industry. It is a violent business that preys on the young and vulnerable, yet one that hides that reality behind a sense of normalcy created by sites like Backpage.com. Removing Visa and MasterCard from the business will raise the bar for pimps and traffickers who seek to place ads, help eliminate a financial incentive to host such ads, and remove the mask of normalcy that has fueled trafficking’s dangerous growth. American Express was previously an option for trafficking ad buyers, but that company removed its card as a payment option earlier in 2015.

Backpage isn't gonna take it any longer. In a Tuesday federal lawsuit, Backpage says Dart's actions and the "credit card companies' acquiescence to his pressure have cut off nearly all revenue."

We're not going to take it...

The suit maintains that Dart's actions violate the Communications Decency Act because "ads are protected speech and that websites are immune from state-law civil or criminal liability" for what their users post. What's more, the site claims in its suit that Dart's actions amount to "an informal extralegal prior restraint of speech."

According to the suit: (PDF)

For over six years, Sheriff Dart has pursued a campaign against online classified advertising websites—first Craigslist and then Backpage.com—demanding they shut down portions of their sites for adult-oriented ads posted by users. At every turn, Sheriff Dart has been stymied by the Constitution, federal law, and court decisions holding that such ads are protected speech and that websites are immune from state-law civil or criminal liability.

Craigslist, however, caved and removed its "adult services" section in 2010.

Backpage says it uses automated and human filtering mechanisms that flag ads connected to illegal activity. The site said it "blocks or removes over a million ads per month" and reports to the National Center for Missing and Exploited Children "any ad that may concern child exploitation."

The suit claims that during backroom meetings between the sheriff and Backpage, Dart had requested that the company require credit cards as the method of payment for adult ads—so the ad poster could be traced. That was something Backpage was already doing.

When the sheriff talks, financial institutions listen

It took just days for Visa and MasterCard to comply with Dart's letters to them requesting they cease doing business with Backpage. The letters, (PDF) on official sheriff's letterhead, "request that your institution immediately cease and desist from allowing your credit cards to be used to place ads on websites like Backpage.com, which we have objectively found to promote prostitution and facilitate online sex trafficking."

The letter challenged the credit card companies to find an ad in the "Adult Escort" section "that is not selling sex."

Dart wrote that the financial institutions "have the moral, social and legal right" to combat prostitution.

Last year, Dart supported proposed legislation that would allow federal prosecutors to shut down sites like Backpage.