OAKLAND, California (BLOOMBERG) - If Mr Elon Musk is able to take over Twitter, his biggest promise is to transform it into a platform for free speech with few restrictions - something he calls "essential to a functioning democracy". But Mr Musk, who is famously sensitive to criticism, has a mixed record on championing the cause.

The 50-year-old billionaire has donated more than US$6 million (S$8 million) to the American Civil Liberties Union in the last five years, making him one of its most substantial donors, and he has discussed free speech on numerous occasions with the organisation's executive director. But in his tweets, public remarks and policies at the businesses he runs, Mr Musk shows little tolerance for speech that is unflattering to him or his companies, or reflects employee criticism of the workplace.

At Tesla and SpaceX, Mr Musk has a long track record of silencing or punishing anyone who goes public with criticism of a project or practice. Workers must sign non-disclosure agreements and arbitration clauses that prevent them from taking their employer to court.

Meanwhile, Mr Musk uses his Twitter account, where he has more than 80 million followers and a fan base he can ignite, to publicly mock others, from a local health official during the early days of the pandemic to Mr Parag Agrawal, Twitter's current chief executive officer.

Mr Musk defined the goal for Twitter at a TED event last week. "A good sign as to whether there is free speech is: Is someone you don't like allowed to say something you don't like? If that is the case, then we have free speech," he said.

But those who have said things that Mr Musk did not like have seen their reputations publicly trashed. Mr Vernon Unsworth, a British caver who helped rescue 12 boys trapped in Thailand, called Mr Musk's efforts to help a "PR stunt" in 2018. Mr Musk retaliated by calling him a "pedo guy". Then he paid US$50,000 to a dubious private investigator to dig into Mr Unsworth's background in Britain and Thailand. He also attempted to depose a reporter, Mr Ryan Mac, who was covering Mr Unsworth's defamation lawsuit against Mr Musk.

That same year, Mr Musk went after Mr Martin Tripp, a worker at Tesla's battery plant in Nevada. Mr Tripp saw himself as an idealist trying to improve the company's operations; Mr Musk viewed him as a dangerous foe who engaged in sabotage and shared data with the press and "unknown third parties".

Tesla's public relations department spread false rumours that Mr Tripp was possibly homicidal and had threatened to "shoot the place up", even though the authorities had already determined that Mr Tripp posed no immediate threat and was not armed.

Another employee was fired six days after he posted a YouTube video of his Tesla Model 3 running into a traffic pylon while using "FSD Beta", an early version of software that Tesla has rolled out to roughly 100,000 people.

And then there is the case of Florida teenager Jack Sweeney, who tracks private jets. A few months ago, Mr Musk reached out to him and offered US$5,000 to shut down the "Elon's Jet" account, Mr Sweeney said. Mr Musk viewed it as a security risk. Mr Sweeney asked for US$50,000, which Mr Musk refused. The billionaire then blocked some of the social media accounts connected to Mr Sweeney.

If Mr Musk were in charge of Twitter policy, he said he believes that people should be blocked only as a last resort, according to his comments at a TED conference. If it is a grey area, his preference would be to leave the content up, he said.

It is difficult to get clarity on the statements Mr Musk makes on Twitter, in part because he largely disbanded Tesla's communications team in the United States and rarely responds to inquiries from the financial press. Several journalists who cover Mr Musk's companies have been blocked by him on Twitter. Mr Musk did not respond to a request for comment via e-mail.

Mr Musk has cited some instances where he believes content on Twitter should be blocked. At TED, he said Twitter should continue to take down content on a geographic basis, as the company "is bound by the laws of the country that it operates in". In Germany, for instance, it is against the law to deny the Holocaust happened, so Twitter hides those tweets in that country.

Mr Musk also said he would like to ban cryptocurrency scammers on the site. The billionaire's persona - and his popularity with crypto investors - has been used to trick people in the past.

And on Thursday, he vowed to defeat spam bots "or die trying" if his Twitter bid succeeds.

While it is unclear if any of Mr Musk's tweets have been removed by Twitter, his account has come under intense scrutiny by third parties, including the US Securities and Exchange Commission, which sued Mr Musk for fraud over his infamous "funding secured" tweet from August 2018. The wording was part of a message saying he was considered taking Tesla private, and it sent the shares surging.

Mr Musk and Tesla ended that dispute by agreeing to pay US$20 million apiece, without admitting wrongdoing. Mr Musk also agreed not to tweet about specific topics without advance approval from a Tesla lawyer.

Twitter's own efforts to moderate speech have drawn recent scorn from Mr Musk. He has said that the company should resort less frequently to banning users. That has led to speculation that Mr Musk could reinstate former president Donald Trump's account if he became Twitter's owner.

"It is always hard when you put humans in charge of making decisions about what speech should be allowed or what speech should not be allowed," said Dr Clay Calvert, who directs the Marion B. Brechner First Amendment Project at the University of Florida. "The one answer for people who object to Twitter's content moderation policies is forming your own company."

Or, in Mr Musk's case, buying Twitter.