Correction Appended

WILMINGTON, Del., Oct. 5 - The Delaware Supreme Court ruled Wednesday that if an elected official claims he has been defamed by an anonymous blogger, he cannot use a lawsuit to unmask the writer unless he has substantial evidence to prove his claim.

That standard, the court said, "will more appropriately protect against the chilling effect on anonymous First Amendment Internet speech that can arise when plaintiffs bring trivial defamation lawsuits primarily to harass or unmask their critics."

At issue was a defamation lawsuit filed last year by Patrick Cahill, a councilman in Smyrna, Del. Mr. Cahill said he needed the identity of a blogger who in a September 2004 posting praised the mayor but said Mr. Cahill was divisive and had "an obvious mental deterioration."

In a second posting, the blogger, named John Doe in the suit, wrote that Mr. Cahill "is as paranoid as everyone in the town thinks he is," according to court records.