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Facebook and Twitter are once again under fire from Congress for allegedly suppressing the views of conservatives.

The tech giants faced accusations from the Senate on Wednesday, a day after Facebook and Google attended another high-profile hearing over the spread of white nationalism on their platforms.

"Not only does big tech have the power to silence voices with which they disagree, but big tech has the power to collate a person's feed in a way that agrees with their political agenda," Ted Cruz, a Republican from Texas and chairman of the Senate Judiciary Committee's Subcommittee on the Constitution, said in his opening statement. "What makes the threat of political censorship so problematic is the lack of transparency."

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The tech giants denied accusations that they're pushing a political agenda.

"Suppressing content on the basis of political viewpoint, or preventing people from seeing what matters most to them, is directly contrary to Facebook's mission," said Neil Potts, Facebook's public policy director. "We want Facebook to be a place where people can discover a greater diversity of news and information than they could otherwise."

Twitter said it had performed a statistical analysis of Congress members' tweets, in preparation for the hearing. The company tracked tweets over a five-week period, and the social network found "no statistically significant difference" between the reach of tweets by Democrats versus Republicans, said Carlos Monje Jr., Twitter's director of public policy and philanthropy for the US and Canada.

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As Republican senators chided the tech companies, Democrats criticized the topic of the hearing. "For decades, Republicans have bashed the supposedly liberal mainstream media in an effort to work the refs," said Mazie Hirono, a Democrat from Hawaii. "This is politics."

Google was also initially set to testify. The search giant had offered to send Max Pappas, its head of conservative outreach, but a source familiar with the situation said the committee rejected that idea. During the hearing, Cruz said the committee would hold a separate hearing specifically focused on Google and political bias.

This isn't the first time tech platforms have been accused of anti-conservative bias. Last September, Facebook COO Sheryl Sandberg and Twitter CEO Jack Dorsey were questioned by GOP lawmakers about a perceived left-leaning agenda. Google CEO Sundar Pichai was also invited to appear, but he declined the invitation. When Pichai did finally testify in December, he too was accused of stifling conservative voices.

President Donald Trump has also criticized the tech giants for political bias. Last August, he complained that Google was suppressing his message by not promoting his State of the Union speech (which turned out to be untrue). He also said that search results for news associated with him was "rigged."

"Google and Twitter and Facebook, they're really treading on very, very troubled territory," Trump said. "And they have to be careful."

Ahead of the hearing, advocacy groups shot down the idea that tech platforms treat conservative content unfairly. "While we certainly agree that online platforms have created content moderation systems that remove speech, we don't see evidence of systemic political bias against conservatives," the Electronic Frontier Foundation said in a blog post.

The Senate hearing comes a day after Facebook and Google testified before the House Judiciary Committee about the spread of white nationalism online. As if to illustrate the scope of the problem, the YouTube livestream of the hearing was flooded with hateful comments, so the Google-owned site shut down the comments section of the stream.

On Tuesday, Cruz livestreamed the hearing on his Facebook page. Unlike with Monday's stream, comments on the broadcast on Cruz's page were civil.

Originally published at 1:13 p.m. PT

Update, 2:52 p.m. PT: Adds more details from the hearing.