I think we can say with confidence, at least 27%.
Rasmussen Reports asked likely U.S. voters whether the government should criminally prosecute those who don’t go along with the global warming scare. This was the precise question:
Should the government investigate and prosecute scientists and others including major corporations who question global warming?
In response, 27% of Democrats called for prosecuting global warming realists. (Remarkably, 11% of Republicans did, too.) Happily, 63% say the debate on global warming is not yet over, while 68% oppose criminal prosecutions. So that 5% difference must represent people who are in favor of freedom of speech.
No wonder that, as Rasmussen also notes, only 20% of Americans say that we enjoy “true freedom of speech,” while 73% “think instead that Americans have to be careful not to say something politically incorrect to avoid getting in trouble.” These are sad times for those who value liberty.