As Cass Sunstein, a professor of law at the University of Chicago, saw himself being skewered on various Web sites discussing his recent book, ''Republic.com,'' he had the odd satisfaction of watching some of the book's themes unfold before his eyes. On the conservative Web site ''FreeRepublic.com,'' the discussion began by referring relatively mildly to Mr. Sunstein's book about the political consequences of the Internet as ''thinly veiled liberal.'' But as the discussion picked up steam, the rhetoric of the respondents, who insisted that they had not and would not read the book itself, became more heated. Eventually, they were referring to Mr. Sunstein as ''a nazi'' and a ''pointy headed socialist windbag.''

The discussion illustrated the phenomenon that Mr. Sunstein and various social scientists have called ''group polarization'' in which like-minded people in an isolated group reinforce one another's views, which then harden into more extreme positions. Even one of his critics on the site acknowledged the shift. ''Amazingly enough,'' he wrote, ''it looks like Sunstein has polarized this group into unanimous agreement about him.'' An expletive followed.

To Mr. Sunstein, such polarization is just one of the negative political effects of the Internet, which allows people to filter out unwanted information, tailor their own news and congregate at specialized Web sites that closely reflect their own views. A ''shared culture,'' which results partly from exposure to a wide range of opinion, is important for a functioning democracy, he argues. But as the role of newspapers and television news diminishes, he wrote, ''and the customization of our communications universe increases, society is in danger of fragmenting, shared communities in danger of dissolving.''

This pessimistic assessment is a sign of just how sharply scholarly thinking about the Web has shifted. In its first years, the Internet was seen euphorically as one of history's greatest engines of democracy, a kind of national town hall meeting in which everyone got to speak. As an early guru of cyberspace, Dave Clark of M.I.T., put it in 1992: ''We reject kings, presidents and voting. We believe in: rough consensus and running code.''