The American left has promoted some astoundingly bad ideas over the past few decades. But when it comes to the safety of American citizens, none has been quite so damaging as the righteous-sounding "sanctuary city" policy.

The noxious scheme started - no surprise - in California. Local cops in Los Angeles and San Francisco were forbidden from stopping suspects merely to inquire about their immigration status.

Things then went from bad to wretched as other cities strove to demonstrate how "humanely" they treated those illegal aliens who were, as we heard ad nauseam, "living in the shadows." Finally, all that humaneness morphed into total insanity when local officials provided sanctuary to some truly bad hombres.

You know the story of Kate Steinle, the 32-year-old woman who was shot to death in San Francisco last week. Illegal alien Francisco Sanchez has pleaded not guilty after initially copping to the murder. A career criminal who reportedly used more than 30 aliases, Sanchez had been deported to his native Mexico five times.

During his most recent visit stateside, he chose San Francisco because it's common knowledge among the criminal class that the City by the Bay goes easy -- very easy -- on illegals.

Democrats have been running away from this putrid mess as fast as possible, not an easy feat while pointing fingers at the same time.

City leaders blame the feds, the feds blame anyone in sight, and the Obama administration, of course, blames Republicans. You know, for not passing "comprehensive immigration reform."

The ideological divide over sanctuary cities was very clear on The Factor this week. Juan Williams actually defended the policy, putting forth the familiar theory that sanctuary encourages illegal aliens to report crimes, thus making those cities safer.

San Francisco Sheriff Ross Mirkarimi parroted that talking point on CNN, saying, "I firmly believe it makes us safer." The sheriff should resign immediately, and Mayor Ed Lee also has to go. Both men have blood on their hands, as do the city supervisors and all the past "social justice" warriors who enabled this gross injustice.

On the other side of the argument, conservative Charles Krauthammer condemned the cities that totally ignore federal law, choosing instead to make their own. His pithy summary: "Sanctuary cities are a disgrace."

Last week's murder was one of those events that captured attention around the country and the world. That is partly because Kate Steinle was gunned down at a tourist spot in one of America's famed cities.

And also because the murder came so soon after Donald Trump railed against criminal illegal immigrants.

If there is one positive, it's that all the attention could lead to action that would truly save lives. Congress needs to pass, and President Obama must sign, a law forcing localities to cooperate with federal authorities. Cities like San Francisco can no longer decide which laws they will enforce, which they will ignore.

Beyond that, any illegal alien who returns to the USA after being deported should face at least five years in the big house. Double that the second time. This absurd and deadly merry-go-round has to stop.

Consider this simple thought experiment: Provo, Utah and Montgomery, Alabama declare themselves "sanctuary cities." Not for illegal aliens, but for deeply religious Americans. Their public schools include Bible study classes and daily church services, while same-sex marriage is banned by local law.

How long before the Justice Department would stomp on those cities and prosecute their leaders? Can you count the minutes?

Yet dozens of cities throughout America are doing essentially the same thing with immigration. They do not follow federal law, they protect dangerous criminals, and they have fomented mayhem and murder.

So let's get on with it, Congress. Do your job by passing "Kate's Law." It will be a fitting way to honor a young woman who was killed by a vicious criminal, and by the policies that enabled him to roam free on the streets of San Francisco. Pass the law. Now.