San Francisco’s liberal mayor declared a “state of emergency” to try to deal with the city’s “nasty streets.”

How did it get so bad?

Journalist Michael Shellenberger’s new book “San Fransicko” argues that it happened because of progressive ideas.

“The town I love is sick,” says Shellenberger.

He came to San Francisco when he was in his 20s to support social justice causes. He still supports those ideas, but “it just went too far.”

In 2014, California politicians decided to end mass incarceration.

It’s a noble goal. America locks up a higher percentage of its people than any other country. Jails are overcrowded. People in jail are more likely to learn to be better criminals than to be rehabilitated.

So California converted many nonviolent felonies to misdemeanors. People who steal less than $950 worth of items are no longer jailed. Proponents said this would divert money from prisons that could go to mental health and drug treatment programs.

But not jailing people who break laws had nasty unintended consequences.

Shoplifters steal right in front of security guards. Police look the other way.

They know if they make an arrest, they’ll face hours of paperwork and the person arrested will just return to the street. Cars are broken into 74 times a day.

“None of us want mass incarceration,” says Shellenberger, who voted for the law to stop jailing people. “But that was a recipe for disaster.”

Because no one is arrested for camping on the street, San Francisco is now filled with tent cities that supposedly house the “homeless.” But most campers are the mentally ill and drug users who choose life on the street. They shoot up or light up in public, confident no one will interfere.

One crack addict said she stays in San Francisco because it is “more lenient.” In other cities, she said, she’d be in jail.

Other cities, like Miami, do treat the homeless differently. “They don’t let people use drugs in public, and they built sufficient homeless shelters,” says Shellenberger.

In San Francisco, new homeless shelters are blocked by progressive activists who argue that everyone deserves an apartment. Yet it costs $700,000 to build one apartment in San Francisco.

A few years ago, I made a video suggesting that the high cost of apartments was a major reason for San Francisco’s tent cities. California’s excessive regulation discourages new construction, so there’s a housing shortage. That keeps rental prices high and leads people to live on the street.

“It’s not true,” says Shellenberger. “If it were true that expensive places made for homelessness, why don’t we see large open-air drug scenes in Carmel?

Why don’t we see large open drug scenes in many fancy neighborhoods? Homelessness is just a function of whether or not you allow people to camp in public or not.”

But if people are homeless, should the government arrest them? The Constitution gives us the right to peaceably assemble.

“People have a right to be outdoors,” I tell Shellenberger. “We don’t have a right to force them off the street if they aren’t directly threatening anybody.”

“We should defend those rights because that’s part of our freedom,” he replies, “but you don’t have a right to shoot heroin at the public park.” There need to be “consequences for people’s behaviors.”

After researching his city’s problems, Shellenberger decided he could no longer identify as progressive. “Progressivism has become the abdication of personal responsibility.”

I think it’s always meant that.

But now parts of San Francisco have become such a sewer that even liberal politicians have changed their positions. The mayor of San Francisco, London Breed, recently declared it’s time to end the “reign of criminals who are destroying our city!”

Not long ago, when protesters shouted, “Defund the police,” Breed cut San Francisco’s policing budget by $120 million.

Now she says her town will be “more aggressive with law enforcement … and less tolerant of all the bulls—- that has destroyed our city.”

Progressive ideas almost always end badly.