Welcome to the national debate over so-called sanctuary cities. “So-called” because, from my vantage point — as the son of a retired cop and someone who has written about immigration for 25 years — there is no such thing as a sanctuary city. 

Now, I know you’re hearing the term quite a bit these days. President Donald Trump has taken on a cause célèbre the idea of cracking down on cities, counties and states that — according to many conservatives — are thumbing their noses at federal immigration law and the agents who are supposed to enforce it. 

Note: In Washington speak, “cracking down” means a combination of public shaming and budgetary threats to strip the local municipalities — cities such as Los Angeles, San Francisco, New York, Philadelphia, Chicago, etc. — of millions of dollars in federal spending. 

And so, recently, Attorney General Jeff Sessions said that he is “urging states and local jurisdictions to comply with these federal laws.” He promised that the Justice Department will require compliance with immigration laws in order for the cities to receive grants through the Office of Justice Programs.

This may or may not be legal. We’re about to find out what the federal courts say, since the localities are certain to file lawsuits to preserve their funding. 

And how exactly are these local and state governments supposedly “thumbing their noses” at their federal counterparts? 

The snub might be a symbolic resolution passed by a city council or county board of supervisors that urges local law enforcement departments not to cooperate with Immigration and Customs Enforcement, or even prohibits such cooperation, unless conditions are met or the individual being sought is — in addition to being in the United States illegally — also a violent criminal.

Or it might simply be a departmental policy put in place by the chief of police to protect his officers and the municipality from lawsuits if things go sideways. Or it might be a policy adopted by the county sheriff, the official who runs the jails, to not alert ICE when releasing an undocumented immigrant back into society — even when ICE places a “detainer” on that individual, requesting that he or she be held until the feds can ascertain legal status. 

But that’s only a courtesy. It’s not a legal requirement that local cops hold on to anyone they suspect might be in the country illegally. They can do it, or not do it. But either way, it’s up to them. They have the power to decide whether or not to cooperate with their federal counterparts. And there is a long history of federal law enforcement not wanting to cooperate with local police, which is also their prerogative. 

To hear Republicans tell it, all this interagency noncooperation has created a bizarro world where we have these “sanctuary cities” that illegal immigrants flock to so they can live happily forever without fear of being detected, detained or deported by ICE agents.

So here’s the problem with that. It’s not true. It’s political fiction. There is no such place. When it comes to the federal government, you can run, but you can’t hide.  

Oh, liberals and lefties want to advance the idea that they have all this power to stand up to the federal government — that is, at least when it is under the control of Republicans — and actually get ICE agents to back down. 

But that’s not happening. Uncle Sam doesn’t care what minor league politicians at the local level say about anything. ICE agents are going to continue to round up and deport illegal immigrants, just like they did — at the rate of 1,000 per day — for the eight years of the Obama administration, even though those same cities had their vaunted sanctuary policies in effect then, too.

That’s odd, isn’t it? The media isn’t asking that question but it should. How did the Obama administration manage to get its hands on more than 3 million people and deport them — even with all these protective sanctuary policies in effect.

I can tell you the answer. It’s because “sanctuary” isn’t made of cast iron. It’s composed of cotton candy. Just add water and it’ll melt away. 

Here’s something that’s missing when we talk about immigration: common sense. Simply put, no matter what you hear from liberals at city hall, or conservatives on Fox News, so-called sanctuary cities are the leprechaun of the immigration debate. They don’t exist. There is no place in America where federal immigration law does not apply, or where local cops are legally obligated to hand over to federal officials — without a warrant — any illegal immigrants they have in their custody.

Someone really ought to explain all this to Attorney General Sessions, before he makes a real mess of things.  

Ruben Navarrette is a syndicated columnist with The Washington Post Writers Group, a member of the USA Today Board of Contributors, a columnist for the Daily Beast and author of “A Darker Shade of Crimson: Odyssey of a Harvard Chicano” (Bantam).