Aug 6 (Reuters) - Elon Musk's social media platform X on

Tuesday sued a global advertising alliance and several major

companies, including Mars and CVS Health ( CVS ), accusing them of

unlawfully conspiring to boycott the site and causing it to lose

revenue.

X filed the lawsuit in federal court in Texas against the

World Federation of Advertisers, Unilever ( UL ) and Danish

renewable energy company Orsted, in addition to Mars

and CVS Health ( CVS ).

The lawsuit said advertisers, acting through a World

Federation of Advertisers initiative called Global Alliance for

Responsible Media, collectively withheld "billions of dollars in

advertising revenue" from X, previously known as Twitter.

It said they acted against their own economic self-interests

in a conspiracy against the platform that violated U.S.

antitrust law.

The World Federation of Advertisers, Unilever ( UL ), Mars and CVS

Health ( CVS ) did not immediately respond to requests for comment.

Orsted declined to comment on Wednesday.

In a statement on Tuesday about the lawsuit, X's chief

executive Linda Yaccarino said "people are hurt when the

marketplace of ideas is constricted. No small group of people

should monopolize what gets monetized."

Ad revenue at X slumped for months after Musk bought the

company in 2022. Some advertisers had been wary of ad spending

under Musk amid questions and fears that their brands would

appear next to harmful content that under prior owners might

have been removed.

The advertising group launched the responsible media

initiative in 2019 to "help the industry address the challenge

of illegal or harmful content on digital media platforms and its

monetization via advertising."

Christine Bartholomew, an antitrust expert and professor at

University at Buffalo's law school told Reuters that lawsuits

alleging unlawful boycotts can face a high bar.

X must show that there was an actual agreement to boycott

joined by each advertiser, Bartholomew said. "Proving this

requirement is no small hurdle" in cases where an agreement

might be implicit, she said.

Even if the case succeeds, X cannot force companies to spend

ad revenue on the platform, Bartholomew said.

The case was filed in the Northern District of Texas and

assigned to U.S. District Judge Reed O'Connor. The district has

become a favored destination for conservatives suing to block

Biden administration policies.

X said in its lawsuit that it has applied brand-safety

standards that are comparable to those of its competitors and

that "meet or exceed" measures specified by the Global Alliance

for Responsible Media.

The lawsuit said X has become a "less effective competitor"

in the sale of digital advertising.

X is seeking unspecified damages and a court order against

any continued efforts to conspire to withhold ad dollars.

Video-sharing company Rumble on Tuesday filed a separate

antitrust lawsuit against the World Federation of Advertisers.