STORY: Angelo Carusone, president of media watchdog group Media Matters for America, said this was a "critical moment" for Twitter.
"If he buys Twitter without any conditions, what's going to happen is Donald Trump, Marjorie Taylor Greene, other extremists, not just in the United States, but in the world, will be will have their accounts restored, and the rules that prevented them from doing certain kinds of extremism and disinformation on Twitter's platform will be eliminated so that they will have free reign," said Carusone.
Discussions over the deal, which last week appeared uncertain, accelerated over the weekend after Musk wooed Twitter shareholders with financing details of his offer.
Under pressure, Twitter started negotiating with Musk to buy the company at the proposed $54.20 per share price.
The deal ends Twitter's run as a public company since its 2013 initial public offering.
Twitter's shares were up about 6% following the news.
Earlier, Republicans in the U.S. House of Representatives welcomed reports that Twitter was poised to agree to a sale to Elon Musk and called on the Tesla chief executive to return former President Donald Trump to Twitter.