The legislation has been three years in the making, and takes aim at the nation's mandatory prison sentences, which many lawmakers and activists warn often do not fit the crime for which the convicted are put behind bars.
"This historic reform bill addresses legitimate over-incarceration concerns while targeting violent criminals and masterminds in the drug trade," said Senate Judiciary Committee Chairman Chuck Grassley, a Republican who introduced the bill with eight Senate colleagues.
"Mandatory minimum sentences were once seen as a strong deterrent. In reality, they have too often been unfair, fiscally irresponsible and a threat to public safety," he added, noting that the policies have cost taxpayers billions of dollars.
Chief targets of the reform are the so-called "three strikes laws" implemented in several states, in which a person is sentenced to a mandatory minimum -- sometimes life in prison -- for a third criminal violation, even if that crime is non-violent drug possession.
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The bill would also retroactively reform the offenses that trigger such enhanced minimum sentences, so that only serious offenses can lead to the mandatory punishments.
More than half the inmates sentenced under California's "Three Strikes You're Out" law are serving sentences for non-violent crimes, according to the Justice Advocacy Project at Stanford Law School.
The measure would also limit solitary confinement for juveniles,enhance punishments for firearm violations by so-called "career criminals," and ease the assigning of inmates to recidivism reduction programs, including work and education programs, drug rehabilitation, job training.