WASHINGTON — It is not too early to identify the sleeper case of the last Supreme Court term. In an otherwise minor decision about a municipal sign ordinance, the court in June transformed the First Amendment.

Robert Post, the dean of Yale Law School and an authority on free speech, said the decision was so bold and so sweeping that the Supreme Court could not have thought through its consequences. The decision’s logic, he said, endangered all sorts of laws, including ones that regulate misleading advertising and professional malpractice.

“Effectively,” he said, “this would roll consumer protection back to the 19th century.”

Floyd Abrams, the prominent constitutional lawyer, called the decision a blockbuster and welcomed its expansion of First Amendment rights. The ruling, he said, “provides significantly enhanced protection for free speech while requiring a second look at the constitutionality of aspects of federal and state securities laws, the federal Communications Act and many others.”

Whether viewed with disbelief, alarm or triumph, there is little question that the decision, Reed v. Town of Gilbert, marks an important shift toward treating countless laws that regulate speech with exceptional skepticism.