One of Hollywood’s most prominent supporters in its effort to secure new online anti-piracy legislation is First Amendment lawyer Floyd Abrams.

Today, Abrams sent a letter to Senate Judiciary Committee leaders giving the greenlight to the Pro IP Act, which is set for markup on Thursday. While opponents, including public interest groups and tech lobbies, argue that the bill threatens free speech, Abrams deems it sufficiently narrow. The law targets sites “dedicated to infringing activities,” and gives the government greater ability to shut down domain names or force payment firms, ad services and search engines to cut off support to such ventures.

“The Protect IP Act is not designed to regulate the Internet,” Abrams wrote. “Nor is it designed to counter the vast array and forms of online infringement, which are subject to various laws already on the books. The Protect IP Act focuses, instead, on a narrow category of entities which are not simply trafficking in some infringing content, or occasionally breaking federal laws, but which are primarily devoted to providing or selling infringing content in the United States.”

Abrams is representing the DGA, AFTRA, SAG, IATSE and the Motion Picture Assn.

For the uninitiated, the words “piracy” and “copyright” can make eyes roll.

But the bill is destined to be this year’s most significant piece of legislation captivating the Hollywood lobby in D.C. this year, and this is just the start to furious rounds of lobbying to make sure it passes. The legislation is aimed at curbing overseas sites that traffic in infringing material and are beyond the reach of current U.S. laws. Although it has significant bipartisan support, not just in the Senate but the House, last week Google’s executive chairman Eric Schmidt suggested that the company would oppose it. The Consumer Electronics Assn. also is critical of the legislation, as is the org Demand Progress. The latter has launched a petition campaign that has been summarily mocked by the MPAA.