An ideologically diverse group of 90 law professors has signed a letter opposing the PROTECT IP Act, the Hollywood-backed copyright enforcement/Internet blacklist legislation now working its way through Congress. The letter argues that its domain-blocking provisions amount to Internet censorship that is barred by the First Amendment.

Jointly authored by Mark Lemley, David Levine, and David Post, the letter is signed not only by prominent liberals like Larry Lessig and Yochai Benkler, but also by libertarians like Post and Glenn "Instapundit" Reynolds.

"The Act would allow courts to order any Internet service to stop recognizing [a] site even on a temporary restraining order... issued the same day the complaint is filed," they write. Such a restraining order, which they describe as "the equivalent of an Internet death penalty," raises serious constitutional questions.

The Supreme Court has held that it's unconstitutional to suppress speech without an "adversary proceeding." That is, a speaker must, at a minimum, be given the opportunity to tell his side of the story to a judge before his speech can be suppressed.

Yet under PIPA, a judge decides whether to block a domain after hearing only from the government. Overseas domain owners (and the speakers who might make use of their websites) aren't offered the opportunity to either participate in the legal process or appeal the decision after the fact. (Affected domain owners may file a separate lawsuit after the fact.) This, the professors say, "falls far short of what the Constitution requires before speech can be eliminated from public circulation."

The law professors also point out that blocking entire domains could "suppress vast amounts of protected speech containing no infringing content whatsoever" if an entire domain is blocked based on finding infringing material on a single subdomain. The Supreme Court has compared such over-broad censorship to "burning the house to roast the pig."

The letter also warns that passing legislation that violates America's free-speech principles will undermine the government's credibility when it tries to promote free speech principles around the world. America's strong support for Internet freedom has "made the United States the world leader in a wide range of Internet-related industries," the professors write. "Passage of the Act will compromise our ability to defend the principle of a single global Internet. As such, it represents the biggest threat to the Internet in its history."