There is a very easy way for Democrats to get major concessions from President Donald Trump on immigration: Give him his wall.

This is the key to a deal codifying the Deferred Action for Childhood Arrivals program, the Obama-era de facto amnesty for a segment of so-called Dreamers. All it takes is giving Trump a plausible start to the wall that the president can then, in his inimitable way, promote as the greatest structure built on a border since Hadrian began his famous handiwork at the northern limit of the Roman Empire in 122.

That the Democrats very likely won’t do this speaks to their irrational aversion to a wall that they can’t view dispassionately any more than Trump can.

It used to be that enhanced security on the border — and yes, a physical structure that in places is effectively a wall — had bipartisan support. The Secure Fence Act of 2006 passed the House by a vote of 283-138 and the Senate 80-19. It called for building roughly 700 miles of double-layer fencing on the border, and no one seemed to believe that the United States had irreparably sullied its reputation.

This wasn’t the first time anyone had thought of a fence, of course. There had been barriers in the San Diego area for a very long time, although not particularly robust ones. Beginning in the 1980s, more serious structures were built. According to The San Diego Union-Tribune, there are 46 miles of fencing overall and 13 miles of double fencing in the San Diego-Tijuana corridor, where there used to be a nightly influx of undocumented immigrants. In some sections, the barriers are 10-feet-tall military helicopter pads indistinguishable from a wall. Again, no one believes San Diego has closed itself off from the world.

But Democrats now find physical barriers on the border offensive, especially if they have enough solidity to be called a wall. Harry Enten of FiveThirtyEight notes that in 2006, almost 40 percent of Democrats supported building a wall. By February of last year, Democrats were against it by 89 percent to 8 percent.

This forecloses the most basic legislative give-and-take. If Nancy Pelosi and Chuck Schumer gave Trump something significant on the wall, they would be able to find their way home — as John Jay said after concluding an unpopular treaty with the British in 1795 — by the light of their own burning effigies. Their voters would scorn them as traitors complicit in the alleged horrid bigotry of Donald J. Trump.

So, they will probably fail to capitalize on a president almost certainly desperate to fall into their arms. Trump cares about one thing above all, and that’s the wall. He knows it was a signature campaign promise. He’d love to be able to go back to his voters with something, with almost anything, that he could brag about as the wall.

This means that even a partial step toward a wall might entice Trump into a deal. On the substance, Democrats who are latitudinarian on immigration should prefer this to any other agreement. The wall is an enhancement of the status quo rather than a departure from it, and even if Democrats go along for now, it’s not clear that it would ever get built.

The Secure Fence Act’s bold promise of hundreds of miles of double fencing petered out under bureaucratic resistance and congressional backtracking. Democrats could easily, if they take the House in 2018 or win the presidency in 2020, defund the wall or otherwise sabotage it. But if they got a lawful amnesty for 700,000 illegal immigrants or even more, depending on the parameters of a deal, that’s never going away.

That’s why restrictionists worried about Donald Trump’s reliability on this issue have an important backstop against a bad deal: the recalcitrance of Democrats who oppose the wall more than they support amnesty for Dreamers.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.