Republicans on the Senate Judiciary Committee, led by Sen. Ted Cruz of Texas, lambasted a top Twitter exec on Wednesday over alleged censorship of conservative users, with Sen. Josh Hawley, R-Mo., accusing Twitter and Facebook of being "anything but transparent."

Cruz led off a hearing of the panel's subcommittee on the Constitution by saying that "a great many people agree that the pattern, the anti-conservative bias and the pattern of censorship we're seeing from big tech is disturbing. The question of remedy is a more complicated one. ... It's a thorny legal question. It's a thorny policy question."

"By almost any measure," Cruz added later in his remarks, "the giant tech companies today are larger and more powerful than Standard Oil was when it was broken up [in 1911]. They are larger and more powerful than AT&T when it was broken up [in 1982]. And if we have tech companies using the power of monopoly to sanction political speech, I think that raises real antitrust issues."

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Twitter Director of Public Policy and Philanthropy Carlos Monje Jr., who sat on a panel with Facebook Public Policy Director Neil Potts, bore the brunt of questioning from Cruz and his colleagues. In his testimony, Monje apologized to Sen. Marsha Blackburn, R-Tenn., for Twitter's decision in October 2017 to block one of her campaign ads, in which she claimed to have "stopped the sale of baby body parts" by Planned Parenthood.

"We made the wrong call," Monje said to Blackburn. "We develop policies governing advertisements that run on Twitter that try to balance allowing our advertisers to promote messages with protecting individuals who did not ask to see that ad. I am sorry."

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Monje ran into more trouble when Hawley asked him about the suspension of recent high-profile accounts, including one for the recently released film "Unplanned," about a Planned Parenthood clinic director-turned-anti-abortion activist.

"The individual who started the @UnplannedMovie account had previously been suspended for breaking our rules and as a result, our automated systems flagged that account and it was taken down for an hour," answered Monje, who attempted to shake off a follow-up from Hawley about whether the company would make its content review protocols public.

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"You say now, you've admitted that there is human involvement in these decisions, that there are, in fact, numerous people involved, there are protocols, but you won't make them public, is that right?" Hawley asked.

"I told you why @UnplannedMovie was banned – was taken down for an hour," Monje shot back, " and I've told you why [conservative writer and radio host] Jesse Kelly was kicked off the platform, and if you want more details, you can ask him."

Hawley then asked Monje if Twitter would commit to a "third-party audit" of "potential bias" on the platform, a question Monje declined to answer.

"This committee hearing again today shows that you’re anything but transparent, and you apparently have no interest in becoming transparent," Hawley said. "This is a huge, huge problem, and I would hope for the sake of the customers you serve and the values you purport to represent, that you would change your behavior and to change your commitments to providing a neutral, unbiased platform for all users in this country."

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Cruz turned his attention to Monje again later in the hearing to ask about restricting the visibility of conservative personalities in search results, a practice sometimes called "shadow banning." Monje denied that Twitter engaged in the practice, but admitted that the company would "downgrade" users' posts "if we have signals that indicate that a person is being spammy, meaning they are using multiple accounts to do the same thing, if they are using automated activity, but we’re not 100 percent sure that they’re breaking our rules, if they’ve been abusive, then what we will do is make it harder for that content to be found."

"When you downgrade a tweet, do you notify the person that you've downgraded?" asked Cruz.

"I'd have to get back to you on that, sir," Monje answered.

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"OK, I believe the answer is no, and if the answer is no, that as far as I can tell is indistinguishable from shadow banning," Cruz fired back. When Monje pointed out that a downgraded user's followers were still able to "find what that person tweeted," the senator responded, "But if it's downgraded so far fewer people see it, that is exactly what is being alleged on shadow banning."

A representative from Google was also scheduled to appear at the hearing, but the committee rejected the company's proposal to send its head of conservative outreach Max Pappas, a former Cruz staffer. Cruz announced that he would hold a separate hearing focused solely on Google at a later date.

Fox News' Guerin Hays and Hillary Vaughn contributed to this report.