Facebook and Twitter defended their handling of U.S. election misinformation at a heated congressional hearing Tuesday where one key senator assailed the platforms for being the "ultimate editor" of political news.

The hearing, the second in less than a month, came with social media under increasing fire from both the left and the right for their handling of political content during a bitter U.S. presidential campaign.

Facebook chief Mark Zuckerberg and Twitter CEO Jack Dorsey testified remotely to the session, called to discuss "censorship and suppression of news articles" and the "handling of the 2020 election" by the platforms.

Senator Lindsey Graham, chairing the Judiciary Committee hearing, warned the CEOs that new regulations are needed to ensure that major platforms are held responsible for decisions on removing, filtering, or allowing content to remain.

"It seems like you're the ultimate editor," the Republican senator said at the opening as he took aim at decisions by both platforms to limit the distribution of a New York Post article claiming to expose malfeasance involving the son of  Joe Biden during the campaign.

"When you have companies that have the power of governments (and) have far more power than traditional media outlets, something has to give."

Graham said the law known as Section 230 that gives immunity to online services for content posted by others "needs to be changed."

- Megaphone for falsehoods -

Democrat Senator Richard Blumenthal also called for reform of Section 230 while rebuking the platforms for what he said was inadequate action on political misinformation from President Donald Trump.

"The president has used this megaphone to spread vicious falsehoods in an apparent attempt to overturn the will of voters," Blumenthal said.

Blumenthal said the big platforms had "power far exceeding the robber barons of the last Gilded Age" and have "profited hugely by strip mining data about our private lives and promoting hate speech and voter suppression."

Republican Senator Mike Lee meanwhile denounced what he called "instances in which your platforms are taking a very distinctively partisan approach and not a neutral one to election related content moderation ... just days before the election."

Both Dorsey and Zuckerberg said they were open to reform on Section 230 as they outlined their efforts to curb harmful misinformation during the election campaign.

"We strengthened our enforcement against militias, conspiracy networks, and other groups to help prevent them from using our platform to organize violence or civil unrest in the period after the election," Zuckerberg said,

He said Facebook removed false claims about polling conditions and displayed warnings on more than 150 million pieces of content flagged by independent fact-checkers.

Dorsey meanwhile said filtering at Twitter was not a result of bias, despite claims to the contrary by conservatives.

In filtering content, "all decisions are made without using political viewpoints, party affiliation, or political ideology," Dorsey said in his testimony.

"Our Twitter rules are not based on ideology or a particular set of beliefs. We believe strongly in being impartial, and we strive to enforce our Twitter rules fairly."

Both platforms have begun limiting the reach of many of Trump's tweets, notably those in which the president rejected his election defeat or questioned the integrity of the voting process.

Twitter (TWTR) and Facebook (FB) have been facing pressure to remove what many see as harmful misinformation around the elections, while also fighting claims of suppression of certain political views.