LOS ANGELES — Prosecutors contend that in August 1975, Sara Jane Olson climbed into a late model Ford Galaxy with two radical pals, drove from the Bay Area to Los Angeles, bought pipe-bomb components from a plumbing store and planted high-power explosives under a pair of black and white police cruisers.

But to win a conviction and send her to jail for life, the prosecution doesn't have to prove she ever left the couch in her San Francisco apartment.

Such is the nature of conspiracy charges — controversial statutes hailed by authorities for their power to prevent and punish crime, but attacked by civil libertarians as vague and unfair laws that amount to guilt by association.

Olson, 54, is charged with conspiracy to commit murder. To find her guilty, jurors must determine that she was part of a group plotting murder — in her case, a wing of the violent, leftist Symbionese Liberation Army — and that someone in the group, not necessarily Olson, took concrete action to advance the plan.

Her 1976 indictment alleges she conspired with five others, SLA soldiers Bill and Emily Harris and three "John Does," to carry out murder. The indictment also lists 13 "overt acts" committed by the alleged conspirators, from visiting the plumbing supply in Los Angeles to maintaining a "repository for bombs and bomb-making materials" in San Francisco.

Prosecutors Michael Latin and Eleanor Hunter are required to prove beyond a reasonable doubt that Olson and the conspirators shared a purpose — killing police officers — and at least one of the overt acts occurred. But they don't have to prove that she lifted a finger.

Olson, formerly known as Kathleen Soliah, admits that she associated with SLA members and was devastated when her good friend Angela Atwood was killed in a shootout with police. But she claims she never joined the group nor participated in any of its crimes. Because she is charged with a conspiracy, her trial will include weeks if not months of testimony about SLA offenses, from murder to car theft, that took place before she allegedly joined.

Since her arrest, she has expressed special contempt for conspiracy laws, saying they unfairly target political activists and are "dangerous to our democracy." Her supporters have suggested the charges are proof prosecutors have little real evidence against her.

"Try her on the actual charges," said Mary Ellen Kaluza, a longtime friend of Olson and a founder of the Sara Olson Defense Committee. "Conspiracy is a pretty insidious tool to get people convicted."

But prosecutors say conspiracy laws — which have always existed in American law and been established as constitutional by the Supreme Court — are a necessary tool to win convictions against very dangerous offenders and that drug dealers, mobsters and violent street gangs, not activists, are the most frequent targets.

"We like conspiracy laws because the more people that get together, the more likely what they plan — a crime — is likely to happen. Conspiracies are particularly dangerous in that way," said former federal prosecutor Laurie Levenson, a professor at Loyola Law School in Los Angeles.

Without conspiracy laws, she said, there would be no hope of dismantling crime syndicates, whether Al-Qaeda or La Cosa Nostra, in which offenses are ordered by a leader and planned with the help of many members, but physically carried out by only one low-level "schlub."

"Who is responsible? Not just the schlub. Everyone," Levenson said.

But opponents of conspiracy laws complain that the statutes have often been used to silence minority voices.

"Clearly one of the risks is that it has been used against unpopular people that you can't convict of crimes otherwise," said Joshua Dressler, a professor of criminal law and procedure at Ohio State and editor-in-chief of the four-volume Encyclopedia of Crime and Justice.

In the 19th and early 20th centuries, labor leaders were often hit with conspiracy charges. In the 1940s, communists and socialists were targeted. And in the 1960s and 1970s, conspiracy defendants were often civil rights activists and anti-war protesters, including child advocate Dr. Benjamin Spock, whose conviction on conspiring to help young men violate draft laws was later reversed.

Although one study shows the conviction rate for conspiracies is the same as for other crimes, experts say that winning a conspiracy conviction is generally easier. Certain hearsay testimony is allowed against conspiracy defendants that would be kept away from the jury in other cases, and evidence of other people's bad behavior — in Olson's case, a murder and a sensational kidnapping by the SLA — is often admissible.

"Legally, the prosecutor still has the burden of guilt, but as a practical matter, the burden is often going to shift to individual members of the accused conspiracy," said Dressler.

The overt acts listed in the indictment are generally easy to prove and not usually crimes in themselves. In Olson's case, for example, the rental of an apartment by Emily Harris is considered an overt act.

"It can be the most minor sort of thing — if they are going to rob a bank and somebody calls the bank to find out if they are open Thursday after six when they want to do the robbery — that's an overt act," said Paul Marcus, a professor at the College of William & Mary's law school and the author of Prosecution and Defense of Criminal Conspiracy Cases.

More challenging for prosecutors is proving that the defendant was in on the criminal agreement. Sometimes prosecutors rely on wiretaps, but often their cases are circumstantial — evidence that the alleged conspirators met and had a discussion.

"In some cases, the agreement can be written down, but normally it's mental — nothing in writing and sometimes, nothing verbal," said Dressler.

In Olson's case, the prosecution will call on SLA kidnap victim turned member Patty Hearst, who claims Olson was an eager participant in several bombings and a fatal bank robbery.

"Often there are one or two of the group who are really bad people and the defendant has to say, 'Yeah, I knew these people, but I wasn't in on it.' It's the old birds of a feather flock together argument and it's really hard to prove a negative," said Marcus.

To many Olson supporters, that smacks of "guilt by association." James Lafferty, the executive director of the National Lawyers Guild in Los Angeles and an Olson supporter, said the conspiracy charges ignored the culture of the 1960s and '70s when membership in left-wing groups was fluid.

"People played around with a lot of organizations that they later found out were no good," said Lafferty. "There was a tendency to flirt with this, that and the other thing."