Here is an excerpt from this article in the New York Times:
The Supreme Court established the First Amendment principles that govern the country’s libel laws in 1964, with its unanimous decision in New York Times v. Sullivan. In that ruling, the court said that public officials had to prove that false statements were made with “actual malice,” meaning news organizations had to have knowingly published a falsehood or published it with “reckless disregard of whether it was false or not.”
The standard, later extended to include public figures, set a high bar for libel and meant that people like Mr. Trump — both a public figure and soon-to-be public official — would have a very, very difficult time winning a libel lawsuit.
Is it correct that the Supreme Court decided that it is illegal for public figures to knowingly state a falsehood? Are there any other laws/cases that shed light on this?