"This is a very important case, not only for California, but for other states as to what it is the appropriate role for the federal government and federal courts to intervene in the operation of state prisons," said Runner.
Citing a long history of federal intervention, prison reformers say the U.S. Supreme Court is poised to uphold the release order, considering lawmakers have dragged their feet for years, not wanting to appear soft on crime.
California's prisons have long been filled to twice their capacity, with a population around 160,000.
"I would fully expect the release is going to go forward. The release of 40,000 prisoners because California has done nothing to mitigate the problems," said Matt Gray, lobbyist and executive director of Taxpayers for Improving Public Safety (TiPS).
Gray hopes California is at least given the opportunity to chose who is released.
"The state still has the chance to do this right," said Gray. "They need to look at each and every offender, look at where they are, do a risk assessment, not just do a wholesale release."
Some high court observers think Justice Anthony Kennedy might be the swing vote, as he has been on other controversial issues.
"He's been unpredictable. He's not a knee-jerk conservative as the right wing of the court has been," said Prof. Michael Vitiello, Univ. of the Pacific, McGeorge School of Law.
California's case will be heard some time after the new term begins in October. In the meantime, lawmakers will have to figure out how they'll pay for all those inmates in these tough budgetary times.