Internet Archive is seeking to intervene in the suit because Backpage doesn't adequately represent its interest in the case, Zimmerman said. For example, the website may opt to settle with the state in order to avoid prosecution or may not adequately pursue the aspects of the law that are of most concern to the Internet Archive.

Internet Archive "wants to get the court to fully engage and fully evaluate the legality" of the state's move to hold service providers liable, Zimmerman said.

The next hearing in the case is set for July 20, when the judge will decide whether to turn the temporary restraining order into a preliminary injunction that would remain in effect while the case is argued.

Tennessee has already passed a law similar to Washington's. New York and New Jersey are considering similar legislation.

The IDG News Service