Eight US lawmakers have withdrawn their backing from anti-piracy laws, amid "blackout" protests on thousands of internet sites.
Two of the bill's co-sponsors, Marco Rubio from Florida and Roy Blunt from Missouri, are among those backing away.
Online encyclopaedia Wikipedia and blog service WordPress are among the highest profile sites to block their content.
The Motion Picture Association of America (MPAA) has branded the protests as "irresponsible" and a "stunt".
The MPAA, Hollywood's primary advocate in Washington and a key supporter of the legislation, is led by former Connecticut Democratic Senator Chris Dodd.
Both bills focus on responding to online piracy, specifically illegal copies of films and other media.
The bills would also outlaw sites from containing information about how to access blocked sites.
The BBC's Jonny Dymond says that with Mr Rubio and Mr Blunt withdrawing their support, the Senate bill - Protect Intellectual Property Act (Pipa) - that had looked likely to pass, now appears to be in trouble.
Mr Rubio is a rising star in the Republican party, and is often suggested as a viable vice-presidential choice for this year's Republican presidential nominee.
Bi-partisan backlash
Republicans and Democrats were among the lawmakers rowing back on Wednesday.
The list of senators no longer backing Pipa includes Mr Rubio and Mr Blunt, and Utah Senator Orrin Hatch, all Republicans, as well as Democrat Ben Cardin of Maryland.
In the House of Representatives, Republicans Ben Quayle of Arizona, Lee Terry of Nebraska and Dennis Ross of Florida said they were no longer supporting the Stop Online Piracy Act (Sopa), joining Pennsylvania Democrat Tim Holden.
Mr Ross tweeted that he was no longer supporting Sopa, because as "a true free marketer, I want IP protected correctly".
In a Facebook posting, Mr Rubio said he and fellow Senators "heard legitimate concerns about the impact the bill could have on access to the internet".
Mr Hatch called Pipa "not ready for prime-time" and said he would remove himself from the bill's list of sponsors.
Blackout on the web
The US news website Politico estimated that 7,000 sites were involved by early Wednesday morning.
Google did not shut down its main search but is showing solidarity by placing a black box over its logo when US-based users visit its site.
Online marketplace Craigslist asks site visitors to contact their representatives in Congress before moving on to the main site.
Visitors to Wikipedia's English-language site are being greeted by a dark page with white text that says: "Imagine a world without free knowledge... The US Congress is considering legislation that could fatally damage the free and open internet. For 24 hours, to raise awareness, we are blacking out Wikipedia."
If users try to access its other pages via search sites, the text briefly flashes up before being replaced by the protest page. However, people have been sharing workarounds to disable the redirect.
WordPress's homepage displays a video which claims that Sopa "breaks the internet" and asks users to add their name to a petition asking Congress to stop the bill.
"The authors of the legislation don't seem to really understand how the internet works," the site's co-founder, Matt Mullenweg told the BBC.
Other net firms that have criticised the legislation decided not to take part in the blackout.
Twitter's founder, Dick Costolo, tweeted that it would be "foolish" to take the service offline.Other net firms that have criticised the legislation decided not to take part in the blackout.
