“Freedom’s just another word for nothin’ left to lose,” singer-songwriter Kris Kristofferson philosophized in his classic blues song, “Me and Bobby McGee,” a half-century ago.

Kristofferson’s tune would be an apt anthem for Gov. Jerry Brown as he winds down his own half-century-long career in politics – especially so since Kristofferson once campaigned for him.

Unless something very unusual happens, Brown will never face voters again. Therefore, with nothing politically to lose, he has the freedom to do whatever he wants.

Brown emitted a very strong clue to his unfettered status last week when he filed a brief with the state Supreme Court in a case affecting public employee pensions, in effect asking the justices to make it easier for state and local governments to reduce benefits.

Brown is supporting appellate court rulings that upheld two provisions of the modest pension reform bill he and the Legislature enacted in 2012, one ending “pension spiking” and the other repealing the ability of public employees to purchase additional retirement credits called “airtime.”

However, Brown appears to go even further, suggesting that the court set aside, or at least severely modify, the so-called “California rule.”

That rule, based on a 1955 state Supreme Court decision, is an assumption that public employee pension benefits, once granted, can never be modified, even for future work.

It is a bedrock issue for public employee unions and the union-controlled California Public Employees Retirement System, as demonstrated when they successfully pressured bankrupt cities not to reduce pension obligations, even though a federal bankruptcy judge said they could do so.

Not surprisingly, any Democratic politician who questions the rule’s legal validity or financial sustainability risks union wrath.

It explains why former Attorney General (now U.S. Senator) Kamala Harris and her successor, Brown appointee Xavier Becerra, have been reluctant to buck the unions by vigorously defending Brown’s pension reform and why the governor, with nothing to lose, decided to do it himself.

A key phrase in one of the appellate court rulings, reinterpreting the 1955 Supreme Court decision, frames the issue that the Supreme Court must decide.

“While a public employee does have a ‘vested right’ to a pension,” Associate Justice James Richman wrote, “that right is only to a ‘reasonable’ pension’ – not an immutable entitlement to the most optimal formula of calculating the pension.”

Were the Supreme Court to agree with Brown and uphold the appellate court rulings that seemingly repeal the California rule, it would be a huge setback for the unions – and a black eye for the local unions that opened the legal door by challenging the pension reform’s abolition of much-abused pension spiking and airtime.

A “reasonable pension” ruling would also be an avenue for local governments, which are now struggling to pay fast-rising “contributions” to CalPERS, to reduce the bite by guaranteeing current benefits for work already performed but reducing them for future work.

Conversely, were the Supreme Court to defy Brown and overturn the appellate courts, the California rule would be enshrined, even mild reforms would be thwarted and the state’s unsustainable pension system could either become insolvent itself or force many local governments into bankruptcy.

Obviously, these are big stakes.