President Donald Trump climbed down on separating families at the border, but the underlying argument is not going away.

The central question isn’t whether we should separate families — even most immigration hawks would prefer to hold families together — but whether migrants should stay in the United States or not.

Trump hopes to salvage his “zero tolerance” policy by holding parents and children together, although the practical and legal obstacles will be formidable. The reaction among immigration advocates has gone from outrage about family separations to consternation about family detention — because their ultimate goal is to let the migrants come into the United States and stay.

This is not to deny that the first iteration of “zero tolerance” was a fiasco. The justifications for the policy from administration officials were different and often clashing, and the effort to pin the whole thing on the Democrats was wildly unconvincing.

Democrats, in particular, didn’t give us any of the rules that have made closing the border to Central American migrants impossible. The Flores consent decree, which makes it difficult to hold kids longer than about 20 days, grew out of a court case decades ago. An anti-trafficking law that prevents us from quickly returning home Central American kids — because they are from noncontiguous countries — was signed into law by George W. Bush.

What is true is that the law makes it impossible to hold Central American parents and children together for any length of time. The children have to be released, and if you are going to keep them together with their parents, the parents have to be released, too. This is the forcing mechanism for waving Central American migrants into the country, and Trump is right that Democrats have no interest in changing it.

When congressional Republicans this week proposed fixing these perversities in the law, Chuck Schumer balked. Democrats don’t want to make it easier for Trump to remove anyone from the country.

It’s easy to lose sight of the radicalism of this position. It’s understandable to oppose deporting an illegal immigrant who has been here for, say, 10 years. He probably has a job. He has a family. He has roots. But these migrants are illegal immigrants who, in some cases, literally showed up yesterday. They have no direct connection to the country and, for most of them, no legitimate claim on it.

The question they pose isn’t whether we are going to let illegal immigrants who are already here stay, but whether we are constantly going to welcome more. It isn’t whether Immigration and Customs Enforcement should hunt people down, but whether it can exclude people. It is, in short, whether we have a border or whether a certain class of migrants can — for no good reason — present themselves to the authorities and expect to be admitted into the country.

Some of these migrants will claim asylum, but these claims are mostly bogus. There is no doubt that they are desperate, and desperate to get into the United States. But they aren’t persecuted back home, even if they fear gangs or a violent boyfriend.

The merits don’t matter under the current system, though. If an asylum-seeker passes a credible fear interview — almost all do — and comes into the United States pending adjudication of his case, it is unlikely that he will ever be seen again. And why not? Who wouldn’t take advantage of that opportunity?

Trump is right to want to end this dynamic and swiftly and reliably deport new migrants, which would be the only sure deterrent against the ongoing influx. But unless Congress acts, it will likely prove impossible, and Trump will still get political opposition, although on changed grounds.

In perhaps the first totalitarian analogy in this new phase of the debate, immigration advocate Frank Sharry said Ted Cruz’s proposal to hold parents and children together would create “family gulags.”

Increasingly for the left, the true enemy is enforcement, and the battle has just been joined.

Rich Lowry is a syndicated columnist. He can be reached via e-mail at: comments.lowry@nationalreview.com.

Rich Lowry