Wikipedia, the online encyclopedia that can be altered by anyone with a computer, has proved remarkably useful for pulling political dirty tricks.

Political operatives are covertly rewriting — or defacing — candidates' biographical entries to make the boss look good or the opponent look ridiculous.

As a result, political campaigns are monitoring the Web site more closely than ever this election year.

Revisions made by Capitol Hill staffers became so frequent and disruptive earlier this year that Wikipedia temporarily blocked access to the site from some congressional Internet addresses.

The pranks included bumping up the age of the Senate's oldest member, West Virginia's Robert Byrd, from 88 to 180, and giving crude names to other lawmakers.

The entry for Democratic Rep. Jim Marshall of Georgia labeled him "too liberal" for his state, in part because of a contribution he received from a political action committee run by Sen. Hillary Rodham Clinton. The man who doctored Marshall's biography now works for his Republican challenger.

In Georgia this week, the campaign manager for a candidate for governor resigned amid allegations he doctored the Wikipedia biography of an opponent in the Democratic primary.

Morton Brilliant was accused of revising the entry for Lt. Gov. Mark Taylor to add his son's arrest last August in a drunken driving accident that left his best friend dead.

The information was accurate and had been in the news. But Brilliant's boss, Georgia Secretary of State Cathy Cox, declared the son's legal troubles out of bounds.

The link to Brilliant was discovered by Taylor's campaign, which immediately accused the Cox camp of engaging in "gutter politics" and demanded Brilliant's resignation.

Some 1,000 volunteer monitors scan changes to Wikipedia's entries to keep them free of obvious partisan editing, factual errors and profanity, said Wikipedia founder Jimmy Wales.

"The beauty of a forum like this is free speech," Wales said. "But we also promote a neutral point of view."

Wales said entries have to meet a standard of newsworthiness and, as a general rule, should not be written by an interested party — either a supporter or an opponent.

But finding out who is writing what on the site is not always easy. Internet addresses can be traced to a computer, but not necessarily to the person at the keyboard. And experts say someone with computer savvy could easily cover his or her tracks.

With more and more Americans getting news and information from the Internet, the stakes are high.

Wikipedia had 25.6 million unique visitors in March, making it the 18th most popular site on the Internet.

Not surprisingly, the Wikipedia entry that has been altered the most is President Bush's.

"Take a deep breath," the site urges readers about to plunge into the passionate political debate.

Other changes are more subtle rewrites of history. Former House Majority Leader Tom DeLay's name has mysteriously started to disappear from the entries of some officials with ties to the embattled Texas politician who is facing a money-laundering trial.

The staff of Rep. Martin Meehan, a Massachusetts Democrat, rewrote his biography to delete a reference to a promise, since broken, that he would serve only four terms.

Wikipedia leapt into the news last year after the journalist and former Kennedy administration aide John Seigenthaler Sr. complained that someone had edited his Wikipedia entry to say that he had been involved in the assassinations of John and Robert Kennedy. The man who posted the false information said it had been a joke.

The flap prompted Wikipedia to adopt stricter controls, Wales said.

However, such oversight is probably minor, said Steven Jones, who teaches communications and technology at the University of Illinois at Chicago.

"Given the sheer size of Wikipedia and the sheer number of entries, it seems impossible that they could police it in an effective way," Jones said.