It seems unbelievable that someone could spend the rest of his life in prison for marijuana possession, but that's the harsh reality for Missourian Jeff Mizanskey.

Because of Missouri's laws regarding "prior and persistent drug offenders," which allow judges to sentence third-time offenders to life in prison for drug charges even if no violence occurred and no guns were involved, Mizanskey has spent the last 20 years behind bars.

He expects to die there.

Mizanskey was arrested in 1993 and was found guilty of possession of five pounds of marijuana and intent to distribute a controlled substance after driving an acquaintance, Atilano Quintana, to a drug deal that turned out to be a sting operation. Though Quintana—the target of the sting—and the other men involved were released from prison years ago, Mizanskey has been locked up ever since.

He had already been busted twice for pot: In 1984, he was caught selling an ounce of marijuana to a relative, who later sold it to an undercover officer. When authorities linked the drug back to Mizanskey, they searched his home and found a half pound of marijuana. He was charged with felony possession of more than 35 grams of marijuana and felony sale of a controlled substance, according to The Riverfront Times.

Seven years later, he was busted again for possession of more than 35 grams of marijuana. The second strike landed him in county jail for 60 days, the Times reports. Missouri now spends roughly $22,000 per year to keep Mizanskey behind bars until the day he dies.

That is, unless Mizanskey's son, Chris, is successful. He has started a petition to convince Missouri Gov. Jeremiah Nixon to grant his father clemency. Chris Mizanskey was only 13 when his father was arrested the last time.

In the time since he has been in prison, Mizanskey has missed the milestones that punctuate growing older. The 61-year-old is now a grandfather to children he may never meet. Between the clemency petition and lobbying state legislators to take up his case, Mizanskey's advocates have tried everything to get his story moving.

The younger Mizanskey writes in the petition:

My dad is, and always has been, a good man. He taught my brother and I all about construction and a good work ethic. He has never been violent and he is a model prisoner. And over the 20 years he has been in that little cell, he has watched as violent criminals, rapists, and murderers have "paid their debts" and left—sometimes just to return a few months later.

Mizanskey’s petition comes as states throughout the country consider legalizing marijuana; medical pot is already legal in 20 states and the District of Columbia, and Colorado and Washington legalized pot last year, with Oregon and Alaska poised to follow. Some 14 states are considering medical marijuana laws, according to the Marijuana Policy Project, while 12 states are debating decriminalizing the drug.

Statistics also show growing acceptance of marijuana: Fifty-five percent of Americans support legalizing marijuana, according to a January NBC News/Wall Street Journal poll.

Attorney Tony Nenninger remembers the first time he met the younger Mizanskey. The two crossed paths during a meeting of a pro-legalization group called Show-Me Cannabis.

"Chris said, 'I'm here because my dad is doing life in prison for pot,' " Nenninger recalls. "I could hardly believe it. It stuck with me."

Following months of research on marijuana laws, Nenninger decided to help Mizanskey with the clemency plea. The document explains the elder Mizanskey's history of nonviolent marijuana-related offenses and details his final run-in with the law. Mizanskey had never served time in state prison until this last bust and had never been convicted of a violent crime; his prior marijuana convictions didn't have aggravating factors. Mizanskey unsuccessfully appealed his conviction in 1995, 1997, and 2011.

Nenninger writes in the clemency request:

I am not aware of any other person in Missouri who is serving a life sentence for non-violent cannabis-only offenses. It is no secret that all recent major polls indicate over 50 [percent] of Americans, including Missourians, favor the complete legalization of adult use of marijuana. We are not asking you to commit to this new majority preference for cannabis legalization, but rather as Governor of Missouri to represent the current population's modern socio-political trends to liberalize marijuana laws in considering the commutation of Jeff's sentence.

Mizanskey's case is well-known in Missouri's legal circles as an example of excessive punishment for nonviolent crime. Kate Hummel, a St. Louis–based attorney, said the state's prior and persistent drug offender law leaves no room to fight such sentences.

“When you have laws like this with no bending room, you get a case like Jeff where it's really unjust,” Hummel said. “It's absolutely unfair.”

While Mizanskey is the only person serving a life sentence for a nonviolent pot offense in Missouri, according to Nenninger, he isn’t alone. A report released by the American Civil Liberties Union last November details 110 examples of nonviolent offenders spending life in prison, most of whom are serving time for drug-related crimes.

As of 2012, there were 3,278 people serving life sentences for nonviolent drug and property crimes in federal prisons, according to data released by the Bureau of Prisons and state departments of corrections—nearly 79 percent of those prisoners are serving for nonviolent drug crimes. Nenninger hopes his work with the Mizanskey family will help those people too.

"To think that somebody is getting this extreme a punishment is terrible,” Nenninger said. "This is the single most striking example of a bad law. It's bad for society.”