HELENA, Mont. - Theodore Kaczynski's lawyer asked a judge to block the prosecution of the Unabomber suspect, saying yesterday that government leaks have jeopardized his chances for a fair trial.

Michael Donahoe said the release of information from unidentified federal sources demonstrates a ''lynch-mob mentality'' on the part of federal investigators.

The result was a ''lethal media blitz'' that ''poisoned the entire population of grand jurors within the United States against Mr. Kaczynski,'' he wrote in his motion.

Donahoe, one of two court-appointed attorneys for Kaczynski, said the prosecution should be halted until the court can decide whether the leaks will prevent a fair trial.

''The government should not be allowed to proceed - before various grand juries throughout the country that have been permanently poisoned by the government's outrageous conduct - in disclosing to the media the highly incriminating nature of evidence taken from Kaczynski's cabin,'' Donahoe said.

''In Mr. Kaczynski's case, the possibility that he could ever be afforded anything that might remotely resemble that (fair trial) process has been forever lost.''

U.S. District Judge Charles Lovell did not immediately rule on Donahoe's petition, which sought a delay in any grand jury proceedings against Kaczynski.

A federal grand jury will hold its regularly scheduled meeting Wednesday in Great Falls and is expected to hear evidence against Kaczynski. The lone charge filed against him thus far makes no mention of the Unabomber attacks that killed three people and injured 23 over the past 18 years.

Lovell scheduled a hearing for 3:30 p.m. Friday to consider whether to pursue Donahoe's motion by calling witnesses, and told both sides to prepare written briefs.

Kaczynski, 53, was arrested April 3 after federal agents, tipped off by his suspicious brother, began searching his mountain cabin near Lincoln.

Leaks about the case began appearing as soon as a search of Kaczynski's home began April 3. Most suggested that evidence collected there tied Kaczynski to the Unabomber bombings.

Donahoe's motion also requested a hearing to question Justice Department officials about what they had done to prevent leaks. He asked Lovell to dismiss a charge of possessing bomb-making materials, and requested the return of hundreds of items taken in a search of Kaczynski's cabin.

Should the government contend it is not responsible for the leaks, Donahoe said he will put reporters on the stand and ask them to identify their confidential sources.

''If past practice is an indicator, the effort will be a futile one,'' said Dick Winfield, who has practiced media law in New York for 30 years.

Winfield said Donahoe's intent may be to scare federal officials into believing they may be discovered as sources.