How did New York Times Co. v. Sullivan make defamation law a constitutional issue?

New York Times Co. v. Sullivan (1964) transformed defamation law from a matter of state common law into a constitutional issue by establishing that the First Amendment and Fourteenth Amendment limit a state’s power to award damages in libel actions brought by public officials against critics of their official conduct.1

Prior to this ruling, defamation was largely governed by individual state laws, which often placed the burden of proof on the publisher to ensure absolute factual accuracy. In Sullivan, the U.S. Supreme Court determined that such strict standards failed to uphold the freedom of speech and of the press required in a democracy, where public debate must be "uninhibited, robust, and wide-open".1

To protect this constitutional interest, the Court introduced the "actual malice" standard, requiring public officials to prove that a statement was made with knowledge of its falsity or with reckless disregard for the truth. Furthermore, the Court asserted that appellate courts have the authority to review the facts in these cases to ensure that local juries do not use minor errors to suppress legitimate criticism of government actions.12