The U.S. Department of Justice has been fighting an extended legal battle since 1998 to enforce a federal law that targets Web sites deemed “harmful to minors.” On Tuesday, it lost again.

This week’s ruling (PDF) by the Philadelphia-based Third Circuit Court of Appeals means Web site operators can continue to relax, at least for now, about the Child Online Protection Act being enforced against them. COPA includes criminal penalties, including fines and six months imprisonment, for anyone found guilty of violating it.

The court concluded that COPA “cannot withstand a strict scrutiny, vagueness, or overbreadth analysis and thus is unconstitutional” and upheld a lower court’s ruling from March 2007 that said the federal law was unconstitutional.

As a side note, because the law was written so long ago, it’s surprisingly limited. It applies only to material delivered “by means of the World Wide Web”–meaning it doesn’t cover peer-to-peer file sharing, the Usenet newsgroups that alarm New York’s attorney general, games like Virtual Hottie 2, those naughty things happening in Second Life, videos watched via a third-party iPhone application, or streaming porn viewed through the VLC, RealPlayer, or Windows Media Player desktop applications.

Still, it’s too early to say that this is the end of COPA. The Bush administration is guaranteed to appeal to the U.S. Supreme Court, which has come up with some surprising rulings in the two times it has already reviewed the law.

That’s not a typo. The Supreme Court has handed down two preliminary rulings, once in 2002 and again in 2004. The first time it sent the case back to the Third Circuit with instructions to broaden its legal analysis beyond the law’s interaction with community standards; the second time it wanted a review of whether “technological developments” have affected the law’s constitutionality.

Now the court seems ready for a final ruling probably by next summer–and the more conservative justices conceivably could assemble a majority to uphold COPA as constitutional.

It could work like this: The Supreme Court’s ruling in 2004 against the Justice Department and in favor of the ACLU commanded a narrow 5-4 majority, with justices Stephen Breyer, William Rehnquist, Sandra Day O’Connor, and (separately) Antonin Scalia dissenting.

The Breyer-written dissent said that COPA places “minor burdens on some protected material–burdens that adults wishing to view the material may overcome at modest cost. At the same time, it significantly helps to achieve a compelling congressional goal, protecting children from exposure to commercial pornography. There is no serious, practically available ‘less restrictive’ way similarly to further this compelling interest. Hence the Act is constitutional.” Scalia went even further.

In the last four years, of course, John Roberts has succeeded Rehnquist and Samuel Alito has succeeded O’Connor, who was often a swing vote on free speech matters. The question for next year is whether the court’s conservatives can pick up a majority, which would uphold COPA as constitutional and breathe life into a decade-old law that everyone else has forgotten about