Bitcoin declined during the hearing, dropping as much as 8.9 percent on the day. The currency fell to $9,951.89 at 12:52 p.m. in New York. It’s down more than 16 percent this week.

Protecting Consumers

Marcus said that he didn’t know which agency might oversee Libra but pledged to work with whatever regulator might step up. He also said that the decision to base the cryptocurrency operations in Switzerland was not a move to evade U.S. regulation.

Marcus, who will appear before the House Financial Services Committee Wednesday, pushed back on some of the criticism, explaining that the token isn’t intended to compete with countries’ national currencies and won’t interfere with central banks’ monetary policy. Additionally, he said user data would be protected.

He also stressed that Facebook is just one of dozens of companies involved in Libra, and said that its control over the coin would be limited.

Defending Facebook

At least one senator -- Republican Pat Toomey of Pennsylvania -- stood up for Facebook, saying that efforts to block the Libra launch were“wildly premature” and misguided. “There are tremendous potential benefits in blockchain technology and cryptocurrencies,” Toomey said. “We should be exploring this.”

Mostly, however, lawmakers chose to question Marcus about the series of scandals that have engulfed Facebook in recent years. That included the company’s agreement last week to pay some $5 billion in a record privacy settlement with the Federal Trade Commission, as well as its role in spreading pro-Trump fake news reports in the 2016 campaign.

Senator John Kennedy noted that “Facebook now wants to control the money supply” and asked, “what could go wrong with that?”

The Louisiana Republican hit the company particularly hard on what he said was a slow and dishonest response when it learned that Russians were using the platform to influence the presidential election. He also took Facebook to task for helping to speed the demise of journalism.