hen Jeffery Pollock ran for Congress last year, he posted his forceful opinions on more than a dozen topics on his Web site, pollock4congress .com, including his support for the federally mandated use of Internet "filtering" software to block pornography in schools and libraries. 

Then he discovered that his own site was blocked by one of those filtering programs, Cyber Patrol.

The experience led Mr. Pollock, a 47-year-old Republican from Oregon, to reconsider his views on filtering. What he once thought of as protection, he said, now looked a lot like censorship. "To mandate the federal government to legislate morality, I find abhorrent," Mr. Pollock said.

Congress apparently disagrees. In December, lawmakers passed a bill requiring federally financed schools and libraries to use a "technology protection measure" like filters to block access to obscene material, child pornography and anything considered to be "harmful to minors." Tucked into an enormous last- minute appropriations bill, the new law bars schools and libraries that do not comply from receiving federal money from a number of sources — including the popular e-Rate program, which has helped to link thousands of public and private institutions to the Internet.

On Tuesday, Mr. Pollock plans to join the American Civil Liberties Union in filing suit in federal court in Philadelphia to overturn the new law, known as the Children's Internet Protection Act. He is part of a group of Internet publishers of similarly blocked sites, as well as libraries and library patrons. The American Library Association is set to file a suit at the same time. Both lawsuits contend that the law violates the First Amendment's guarantees of freedom of speech.

It would be the third time a federal law that sought to shield children from the Internet's wild side faced scrutiny in a Philadelphia court. So far, the government has not fared especially well. The civil liberties union and other groups successfully fought the 1996 Communications Decency Act. Civil liberties groups have also won a preliminary round in their challenge to the Children's Online Protection Act, a more narrowly drawn follow-up law; a federal judge has blocked enforcement of that law until a full trial can be held.

In the cases to be filed on Tuesday, groups contend that even the best filtering programs are still rough tools that tend to block legitimate sites and let objectionable sites slip through. That means sites with constitutionally protected material, like Mr. Pollock's, and other often- blocked sites, including those of Planned Parenthood, hit a digital dead end.

The blocking of Mr. Pollock's site was a mistake, said Susan Getgood, who heads the education division for SurfControl, the maker of Cyber Patrol. Ms. Getgood said it happened because home versions of the product blocked any site from computer servers that were also hosts of pornography sites. Many companies that are hosts of Web sites for the general public are also hosts of pornography sites, and Mr. Pollock's host was on Cyber Patrol's list even though there was nothing on his site that should have triggered the blocking process. Ms. Getgood said the flaw had been fixed in the coming version of her company's product.

She said her company opposed the filtering bill because schools and libraries should be free to choose filtering as an option. "It's about choice," Ms. Getgood said. "We make software, not policy."

Supporters of the new law contend that its critics overstate the limitations of filtering programs. "There's a tremendous amount of flexibility and customizability that we didn't see in filters five or six years ago," said Donna Rice Hughes, an anti- pornography activist who is a consultant to a filtering company called FamilyClick.

"Some are better than others," Ms. Rice Hughes said, but over all, the software "has greatly improved over the years." 

The proponents of the law contend that protecting children from the seamy underside of the online medium is the kind of compelling societal interest that justifies the imposition of filters. Besides, Ms. Rice Hughes said, "if they don't want to use protection tools, fine. Then they don't get federal money for Internet access."

In a letter to Congressional colleagues last year seeking support for the bill, one of the sponsors, Representative Ernest Istook, Republican of Oklahoma, wrote that his proposal had become a target for civil libertarians.

"They treat it as `someone else's problem,' " Mr. Istook wrote, "and falsely label it `censorship' if they're not permitted to expose our children to the very worst things on the Internet, using federal tax dollars to do so." 

Ann Beeson, a lawyer for the American Civil Liberties Union, said the new law perverts the e-Rate program, which was intended to help bridge the so-called digital divide in America between rich and poor, and urban and rural. Ms. Beeson said the new act would create a "dumbed- down version of the Internet" for students at poorer schools, which have the fewest alternatives to federal aid.

Judith F. Krug, director of the library association's office for intellectual freedom, warned that if the law went into effect as scheduled in April, "it's going to be detrimental to the very people that Congress is trying to protect" with programs like e- Rate.

Nancy Willard, an expert in computers in education at the University of Oregon, said the law gave too much power over the education process to private companies with motives and agendas that were unclear. "The responsibility and the authority for decision making about which sites fit or do not fit into these specific filtering categories have been transferred from educators and librarians to companies that are totally unaccountable," Ms. Willard said. "What we have here is a situation where there is a great potential for bias." 

She said that since the blocking lists maintained by the companies were generally considered trade secrets, determining what was blocked became a cumbersome, hit-or-miss proposition. Some of the filtering companies also run advertising for the captive audience in schools and even offer marketers a peek at the patterns of their students' Web wanderings, Ms. Willard said.

One librarian who has not bought filters so far said she preferred training students and parents to use the Internet safely and wisely. "People would have you believe that bad behavior in libraries started with the Internet, but it didn't," Elizabeth Heath of the community library in Seabrook, N.H., said.

Although the technologies are new, the argument over the proper limits of speech is old, and fundamental to the founding of the American experiment in government. The civil liberties argument goes back to the time of the founders, who believed that the free exchange of information was an essential element of democracy. And in the 1998 federal court decision that declared the Communications Decency Act to be unconstitutional, Judge Steward Dalzell wrote, "It is no exaggeration to conclude that the Internet has achieved, and continues to achieve, the most participatory marketplace of mass speech that this country — and indeed the world — has yet seen."

Mr. Pollock, whose site remains posted despite his defeat in the November election, said he was surprised to find himself allied with the civil liberties union, a group he had little sympathy with in the past. But he said thoughts of the intentions of the framers guided him.

"I'm sure that the founding fathers would not want this to occur," he said.