Foes of the controversial Stop Online Piracy Act are rallying opponents ranging from Internet engineers to First Amendment scholar Laurence Tribe ahead of an expected committee vote on the legislation this week.

Their aim is to sway the 39 members of the House Judiciary committee, which oversees copyright law. The panel's chairman is Lamar Smith of Texas, Hollywood's favorite House Republican and the principal author of SOPA, which has drawn what may be an unprecedented public outcry from Internet users and companies including Facebook, Twitter, Mozilla, eBay, and Google.

Tribe, a high-profile Harvard law professor and author of a well-regarded treatise titled American Constitutional Law, concluded recently that SOPA violates the First Amendment because it allows government action to effectively "disappear" an allegedly piratical Web site from the Internet. (See CNET's FAQ on SOPA.)

SOPA's free speech risks were illustrated last week when the Department of Homeland Security abruptly decided to relinquish control of a hip-hop music blog called DaJaz1.com, which it seized in November 2010. Even though DaJaz1 was never found liable for a single instance of copyright infringement, its domain name had nevertheless been inaccessible for over a year.

Under SOPA, "an entire Web site containing tens of thousands of pages could be targeted if only a single page were accused of infringement," Tribe warns. "Such an approach would create severe practical problems for sites with substantial user-generated content, such as Facebook, Twitter, and YouTube, and for blogs that allow users to post videos, photos, and other materials."

Tribe's First Amendment analysis is being buttressed by action alerts and exhortations for Americans to call their members of Congress from Web sites including FightForTheFuture.org, AmericanCensorship.org, and IWorkForTheInternet.org (which lets you upload your very own I-oppose-SOPA photo). Other groups are urging support for a SOPA alternative called the OPEN Act, which, as CNET reported last week, would take a follow-the-money approach by by targeting only Internet ad networks and "financial transaction providers" such as credit card companies.

In addition, the venerable Internet Society said today that it's "deeply concerned" about SOPA. The Public Interest Registry, which operates the .org top-level domain registry, has similar concerns. And a new, non-partisan Congressional Research Service report notes that the motion picture and sound recording industry's value-added share of GDP is a mere 0.4 percent -- far less than the Internet and consumer electronics industry.

SOPA represents the latest effort from the Motion Picture Association of America, the Recording Industry Association of America, and their allies to counter what their members view as rampant piracy on the Internet, especially offshore sites such as ThePirateBay.org. The measure would allow the Justice Department to seek a court order to be served on search engines, Internet providers, and other companies forcing them to make a suspected piratical Web site effectively vanish from the Internet.

While much of the criticism of SOPA has come from liberal groups including Public Knowledge, New America Foundation, and Demand Progress, not-so-liberal opponents want to make sure that their concerns are heard as well.

The advocacy group Americans for Limited Government typically focuses on property rights, school choice, the Second Amendment, and other topics dear to the hearts of conservative and libertarian activists.

In an op-ed piece today, the group's president, Bill Wilson, argues that SOPA has morphed into an Internet "kill switch." It would establish the "the precursor to a taxpayer-funded 'thought police,'" he warns. Another ALG essay likens SOPA to China's efforts to restrict and control the Internet.

"While many of the critics of SOPA have also been critical of copyright enforcement," says Berin Szoka, president of the free-market TechFreedom, not all groups who have expressed concerns about the legislation are.

TechFreedom is "supportive of copyright," Szoka says, but has concerns that the bill is being rushed to a committee vote without even one hearing with "witnesses who can speak from expertise to the engineering problems raised by SOPA." (Rep. Dan Lungren, the California Republican who heads the House cybersecurity subcommittee, has expressed similar worries, and the libertarian Competitive Enterprise Institute says SOPA should be changed.)

The same Internet engineers who raised concerns about a similar Senate bill called Protect IP earlier this year now say that SOPA is just as bad. In a new letter (PDF) to key House and Senate members, they reiterate their concerns and respond to a paper (PDF) published last week by the Information Technology and Innovation Foundation that defends SOPA's approach.

Their letter was authored by Steve Crocker, a longtime member of the Internet Engineering Task Force; David Dagon, a post-doctoral researcher at Georgia Institute of Technology; security researcher Dan Kaminsky; Verisign Chief Security Officer Danny McPherson; and Paul Vixie, chairman of the Internet Systems Consortium and principal author of popular versions of the BIND DNS server software.

ITIF's own paper was written by Dan Castro, an analyst who hasn't worked on Domain Name System (DNS) technology, policy, and security issues, including DNSSEC, which is intended to improve the security and reliability of the DNS. Without mentioning Castro by name, the letter's authors say that ITIF lacks "subject matter expertise and experience." They conclude:

Our critics have failed to address our concerns about DNSSEC, and instead describe how older DNS technology can accommodate their plans. The impossibility of the DNS redirection described by SOPA and PIPA should be addressed, but we must caution that "non-answering" brings other harms. There is no support in the DNSSEC protocol for "authentic lies," even if government mandated.

The separate analysis from Tribe, who has argued dozens of cases before the U.S. Supreme Court, acknowledges that Floyd Abrams, a former MPAA attorney and First Amendment lawyer, has concluded (PDF) that SOPA is perfectly constitutional. Tribe's rebuttal: Abrams "does not come to grips" with key portions of SOPA and even he acknowledges that the bill can lead to the "suppression of entirely lawful, protected speech."

Tribe's final verdict on SOPA: "Its constitutional defects are not marginal ones that could readily be trimmed in the process of applying and enforcing it in particular cases. Rather, its very existence would dramatically chill protected speech by undermining the openness and free exchange of information at the heart of the Internet. It should not be enacted by Congress."

Update Monday evening: Here's our followup story confirming the markup and reporting on a new version of SOPA. We also received a response from ITIF's Daniel Castro, saying: "I fully acknowledge the expertise and experience of Steve Crocker, Dan Kaminsky, Danny McPherson, and Paul Vixie. But they say that they first created DNSSEC in response to a request from the U.S. government, so why can't they modify it now upon a new request from the U.S. government? I would encourage them as experts to offer up a recommendation on the best way to bring DNSSEC up-to-date to deal with the government's current requirements, specifically to make sure the DNS does not facilitate criminal activity on the Internet."