In a bipartisan 388 to 25 vote, the House of Representatives approved legislation on Tuesday to make it easier for states to prosecute websites that facilitate prostitution and sex trafficking—including trafficking of underage girls. But critics say that the legislation, known as the Allow States and Victims to Fight Online Sex Trafficking Act (FOSTA), could undermine a key legal protection for free speech online.

The House bill modifies Section 230 of the 1996 Communications Decency Act, which provides website operators with broad immunity for hosting third-party content. Digital rights groups argue that Section 230 made today's innovative and free-wheeling Internet possible because sites don't have to worry about getting sued if their users post content that violates the law.

"FOSTA would punch a major hole in Section 230, enabling lawsuits and prosecutions against online platforms—including ones that aren’t even aware that sex trafficking is taking place," the Electronic Frontier Foundation wrote on Tuesday.

A big concern here is that websites could become less active about moderating their sites because doing so could make them more aware of—and therefore more liable for—objectionable material.

As we explained last September, the fight over online sex trafficking legislation is really a fight about one site—Backpage—that has long been a leading destination for advertising commercial sex services, including those involving underage and trafficked women. Victims' rights groups have argued that Backpage profits off the slavery of young women and that Section 230 makes it too easy for Backpage to avoid responsibility for this.

Courts have thrown out civil lawsuits and state criminal charges against Backpage due to Section 230. Section 230 does allow the federal government to bring criminal charges, but so far no federal case has been brought against Backpage.

Legal pressure caused Backpage to shut down its adult services section last year. However, the site still appears to be a magnet for adult services. Ads on Backpage's "women seeking men" sections look a lot more like ads for prostitution services than conventional dating ads.

At the same time, some critics say that shutting down Backpage wouldn't actually help trafficking victims.

"Allowing Internet platforms on which sexual services are brokered to thrive may be key to apprehending traffickers and recovering victims," wrote Notre Dame legal scholar Ariel Levy in a paper last year. She pointed out that shutting down a site like Backpage is unlikely to prevent pimps from forcing women into sex work—it will simply push the activity underground, where it will be harder for law enforcement to monitor.

Action now moves to the Senate, which has been working on companion legislation called the Stop Enabling Sex Trafficking Act. Civil liberties champion Ron Wyden has put a hold on that legislation, which could slow its passage. But the bill has dozens of cosponsors, and House passage will increase pressure on the Senate to follow suit.