The rule, generally held to be unconstitutional in the United States, that free speech may be abridged if the negative reaction of the audience would cause mayhem.
The phrase "heckler's veto" was first used by the Supreme Court in Brown v. Louisiana (1966), but the idea was recognized by US law at least as early as 1949. In Terminiello v. City of Chicago, the majority opinion read in part:
Freedom of speech, though not absolute, is ... protected against censorship or punishment, unless shown likely to produce a clear and present danger of a serious substantive evil that rises far above public inconvenience, annoyance, or unrest.
In other words, the mere fact that stating your opinion will piss a lot of people off is not sufficient reason to censor you. The Ku Klux Klan is allowed to march in predominantly Jewish neighborhoods, even though tax dollars will be needed to maintain order -- and the Klan may not be charged a fee for that protection. You can say publicly that the emperor has no clothes, no matter what his approval rating.
SOURCE: "Heckler's Veto" by Ronald B. Standler, http://www.rbs2.com/heckler.htm