WASHINGTON (Reuters) - The U.S. Senate may vote as soon as Tuesday on a bipartisan proposal that would make it easier to penalize operators of websites that facilitate online sex trafficking, according to four sources familiar with the matter.

Allowing a vote on the legislation would deal a major blow to the U.S. technology industry, which has vigorously opposed the measure, saying it would thwart digital innovation and lead to endless litigation.

The proposed law was prompted in part by frustrations over failed attempts to shut down the website backpage.com, which many law enforcement officials have accused of facilitating the exploitation of especially women and children to perform sexual services against their will. The company has long denied the allegations.

The Senate is expected to pass an annual defense policy spending bill this week, a process that will include consideration of amendments, some of which have yet to be publicly disclosed.

Among the amendments being considered is the Stop Enabling Sex Traffickers Act, which would change a section of a decades-old law that shields companies such as Facebook Inc, Amazon Inc and Alphabet Inc’s Google from liability for content posted by their users, according to a Senate aide and three technology industry sources.

The sources, who requested anonymity to discuss ongoing private conversations, cautioned on Monday that the situation was fluid and the Republican Senate leadership might decide against allowing a vote on the amendment.

But they also said it was likely to pass if there was a vote, a possibility that has alarmed internet companies. The underlying defense bill is considered must-pass legislation but would need to be reconciled with the House of Representatives and signed by President Donald Trump before becoming law.

Republican Senator Rob Portman introduced the online trafficking bill in August and has gained support from more than a fourth of the Senate.

Kevin Smith, a spokesman for Portman, declined to comment, saying only that the senator was focused on building more support for the measure.

The legislation would allow state attorneys general and victims of sex trafficking to pursue legal action against social media networks, advertisers and others who fail to keep exploitative material off their platforms.

Technology companies say the changes to Section 230 of the Communications Decency Act would impose impossible demands on online publishers and quash innovation. But some lawmakers and attorneys have argued that the statute has been too broadly interpreted to give companies too much leeway in avoiding responsibility for harmful content.