WASHINGTON (Reuters) - Republican senators on Wednesday said Alphabet Inc’s Google, Facebook Inc and Twitter Inc discriminate against conservative viewpoints and suppress free speech, suggesting anti-trust action could be a solution.

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Senator Ted Cruz, who chairs the Senate Judiciary subcommittee, said many Americans believe big tech firms are biased against conservatives and pointed to some anecdotal examples. While no one wants “government speech police,” he said there are other remedies.

“If we have tech companies using the powers of monopoly to censor political speech, I think that raises real antitrust issues,” Cruz said at a U.S. Senate Judiciary subcommittee hearing.

Facebook, Twitter and Google denied their platforms are politically biased, and Democratic lawmakers said there was no evidence to back Republican bias claims although Democrats have criticized the firms on other grounds.

The Senate hearing was a sign that Republicans do not intend to relent in their year-old campaign against the tech companies. Last month, U.S. President Donald Trump again accused social media firms of favoring Democratic opponents without offering evidence.

“We do have a political bias issue here,” Republican Senator Mike Lee said.

Senators also raised the prospect that Congress could remove protections under Section 230 of the Communications Decency Act that give online platforms broad immunity for what users post.

Senator Mazie Hirono, the top Democrat on the panel, said Republicans claims are based on “nothing more than a mix of anecdotal evidence... and a failure to understand the companies algorithms and content moderation practices.”

Democratic presidential candidate Senator Elizabeth Warren said Facebook last month removed ads her campaign placed calling for Facebook’s breakup. “I want a social media marketplace that isn’t dominated by a single censor,” she said.

Carlos Monje, Twitter’s public policy director, said the site “does not use political viewpoints, perspectives or party affiliation to make any decisions, whether related to automatically ranking content on our service or how we develop or enforce our rules.”

Facebook public policy director Neil Potts said the company “does not favor one political viewpoint over another, nor does Facebook suppress conservative speech.”

Senator Josh Hawley told the firms they are not being transparent in how they make decisions. “This is a huge, huge problem,” he said.

Hirono said, “We cannot allow the Republican party to harass tech companies into weakening content moderation policies that already fail to remove hateful, dangerous and misleading content.”

Google was disinvited over a dispute about whether it offered an executive senior enough to testify. The panel left an empty chair for Google. Cruz said he plans a future hearing to address what he called “Google’s censorship of free speech.”

Google said in a written statement submitted to the committee that it works to ensure “our products serve users of all viewpoints and remain politically neutral” but it acknowledged that “sometimes our content moderation systems do make mistakes.”