LOS ANGELES (Reuters) - The parents of a college student stabbed to death 2 1/2 years ago sued former California Governor Arnold Schwarzenegger on Thursday over his decision to reduce the prison term of a politically-connected assailant convicted in the slaying.

Some legal experts said the lawsuit has little chance of success because California governors historically have enjoyed wide latitude under the law to commute sentences of convicted criminals.

Esteban Nunez, the son of former California Assembly Speaker Fabian Nunez, was sentenced to 16 years in prison after pleading guilty to voluntary manslaughter and assault with a deadly weapon for his role in an October 2008 drunken brawl near San Diego State University that led to the fatal stabbing of Luis Santos, a 22 year-old student.

Schwarzenegger, a Republican, shortened the sentence to seven years in one of his last acts as governor on January 2.

Santos' parents, Fred and Kathy, are seeking through their lawsuit filed in Sacramento to have a California court overturn Schwarzenegger's commutation and force Esteban Nunez to serve his full sentence.

"They realize it could be a long battle testing new waters, but they're definitely ready to go the whole distance," Nina Salarno Ashford, a lawyer for the Santos family, told Reuters.

Daniel Ketchell, an aide to the former governor, declined to comment on the lawsuit.

Esteban Nunez was 19 when he and his friends, angry at having been turned away from a fraternity party, attacked Santos on the street. He later admitted to stabbing one of Santos' companions in the stomach. That person survived.

Fabian Nunez is a prominent Los Angeles Democrat whose six years as a state legislator included four years as Assembly speaker. A political ally with Schwarzenegger on a number of issues, he left office in late 2008.

In commuting his sentence, Schwarzenegger cited the fact that Esteban Nunez had no criminal record before the fight, and that his co-defendant, Ryan Jett, who received the same 16-year sentence, had inflicted the fatal wound to Santos, and had prior felony convictions.

Jett pleaded guilty to the same charges as Nunez.

Santos' parents have complained that Schwarzenegger never consulted with them before announcing the commutation.

Ashford said she could gain leverage in the lawsuit against Schwarzenegger from a change to the California Constitution from a 2008 voter-approved ballot measure, known as Marsy's Law, that gave increased power to crime victims.

"We're not saying that (Schwarzenegger) doesn't have the power to commute a sentence, but he does not have the power to violate the Constitution and ... there were mandated guidelines that he needed to follow," Ashford said.

Among those are the expectation that the victim's family will be consulted before making a decision to release a convict, according to the lawsuit.

Laurie Levenson, a professor at Loyola Law School, said that she was doubts the Santos family will prevail.

"It certainly would be groundbreaking," she said. "It's certainly an uphill battle."

She added Marsy's Law is light on enforcement mechanisms for contesting commutations and other gubernatorial actions.

Schwarzenegger, capitalizing on his popularity as a Hollywood action hero, was elected governor in 2003. Term limits prevented him from seeking reelection in 2010, and he was succeeded this month by Democrat Jerry Brown.

(Reporting by Alex Dobuzinskis: Editing by Steve Gorman)