Lawmakers are trying to pass a landmark bill to halt child trafficking, but congressional aides say it is facing resistance from big tech companies that have launched a stealth campaign to fight the legislation.

The SAVE Act is a bill targeting online advertisers that facilitate child prostitution. The Senate version would allow the federal government to shut down advertisements for child prostitutes and, upon conviction, send owners of websites that run the ads to prison for up to 10 years.

Both human trafficking and forced prostitution are already illegal. But what Sen. Mark Kirk and Rep. Ann Wagner, who are pushing the initiative in the Senate and House, respectively, seek to do is create liability for those helping make the connection between pimps and johns.

In particular, lawmakers are targeting websites like Backpage.com, an online classifieds site that receives some 70 percent of the revenues related to prostitution advertising in the United States. In 2013, the media research organization AIM Group found that annual revenue from online prostitution advertising totaled $45 million. Under the SAVE Act, Backpage.com would become more accountable for the ads that run on its site, and its owners could face jail time if they violated the regulations.

“The internet can't be a safe haven for criminals. Selling our kids for sex is modern-day slavery and is being done online every day by companies like Backpage.com,” Kirk told The Daily Beast.

But aides to Kirk and Wagner say big tech companies have quietly lobbied in private against the bill, telling Hill staffers they are concerned the legislation could create liability for them as well. More broadly, the companies say they object to the proposed law on the principle that the Internet should remain relatively free from regulation.

“The Googles of the world are in a tough spot. They’re not going to speak out publicly against a human trafficking measure. But they also are opposed to it,” said a Wagner legislative aide. According to the aide, negotiations with tech associations usually lead to suggestions that legislation enhance penalties for pimps or johns, rather than online advertisers.

Mike Zaneis, the executive vice president of public policy at the Interactive Advertising Bureau, which represents dozens of Internet companies, including Google and Yahoo!, stressed that while his group supports efforts to end the exploitation of children, it fears that the bill would threaten the “integrity of the Internet.”

“[A]s currently drafted, the bill raises serious First Amendment concerns and would classify many innocent web publishers and advertisers as criminals,” Zaneis told The Daily Beast.

But advocates for the bill say they are confused about what big companies are specifically concerned about. The behind-the-scenes discussions, they say, have mostly been in the abstract—tech companies simply didn’t want to open the door to laws targeting content on the Internet, even if it was to hinder child trafficking.

“It’s not something you can even rebut, in terms of something that may or may not happen in the future,” said the Wagner aide. The anti-regulatory attitude, he said he suspected, was born from the fight over the SOPA anti-piracy proposal, which caught tech companies off-guard and exposed them as ill-equipped for a legislative battle.

This rigid objection to regulation is unacceptable in the face of such a serious problem, say the bill’s supporters.

“We can’t have a hands-off philosophy on the Internet anymore,” said a senior GOP aide. “The slippery slope argument is a way of keeping the hands-off-the-Internet-entirely philosophy going.”

Tech companies and associations such as Google, Yahoo!, the Internet Association, and the Internet Service Providers Association all declined requests for comment for this article.

A version of the SAVE Act has already passed the House overwhelmingly, 392-19. The Senate version, which also requires those who place adult advertisements to provide a valid phone number and credit card number, has not yet come up for a vote.

Groups that have spoken out against the Senate bill include the Association of Alternative Newsmedia, which expressed concern about the burden of the Senate bill’s record-keeping requirement, the Center for Democracy and Technology, which flagged free expression concerns, and the libertarian magazine Reason, which saw the potential for these registries to be abused by government and law enforcement officials.

While congressional aides say Internet Service Providers and tech companies are wary of the bill, both House and Senate backers insist that they will work with them to help avoid potential liability for good actors.

“I am certain we can protect ISPs from prosecution in the SAVE Act and partner with them to stop sex traffickers and the lengthy chain of garbage who make millions from selling children for sex. The SAVE Act has them in the cross-hairs,” said Kirk.

Added the Wagner aide, “Internet companies are huge innovators, they’re huge job creators, and they’re really forces for good…we wanted to make sure we didn’t impact these innovators…so we really worked hard to narrowly tailor [the legislation].”

But not all anti-trafficking advocates agree that the SAVE Act is the right approach and prefer instead legislation that would distribute federal money to states and localities to stop human trafficking.

Malika Saada Saar, the executive director of the Human Rights Project for Girls, praised tech companies for their efforts in other avenues to reduce child trafficking and said she doesn’t think going after advertisers as the best strategy to reduce child trafficking.

“We live in an era where it’s just not going to be possible to shut down these websites,” she said. “I understand why people want to go after the websites. I’m more interested in going after a culture which allows people to purchase children with impunity…my belief has been that the way to change is to go after demand.”