The law "will force, by threat of felony prosecution, websites and others to become the government's censors of users' content," Backpage alleges in its challenge.

Holding online service providers liable runs contrary to the immunity carved out in the federal Communications Decency Act, specifically in , both suits assert.

"The appropriate way to combat illegal speech online is to prosecute the people who are engaged in bad conduct," said Matt Zimmerman, a senior staff attorney at EFF.

In a joint on Backpage's suit, the law's sponsor, Senator Jeanne Kohl-Welles, a Seattle Democrat, and Seattle Mayor Mike McGinn called the classifieds site "an 'accelerant' of the commercial sexual exploitation of children."

"We know, based on the experience of multiple online and print forums for escort services, that the only way to be sure that Backpage.com isn't being used to sell children for sex is to verify the ages of the people advertised on their site," the two said.