Not that surprising, really. Take an unusually unpopular candidate, have him make the wall his signature issue, then sit back and watch the numbers deteriorate.

What’s driving the drop? Partisanship, mainly. A year ago, when most people still weren’t taking Trump seriously, their view of a border wall was based on the merits of the idea. A year later, with Trump having become the GOP nominee for president, their view is colored by their opinions of him and the larger immigration politics he’s been practicing. Here’s the partisan split last September:

Independents split evenly and more than a third of Democrats were onboard with the wall. Today?

Indies now oppose the wall and the bottom has dropped out among Democrats. Now that “the wall” has come to mean “Trump,” it’s much easier for border-hawkish Dems to decide that it’s a stupid idea after all. This isn’t the only poll this year to show support for the wall declining, either. Last September, Pew found the public evenly split on the idea of building one, 46/48. Six months later, in March of this year — after Trump had piled up some primary victories — support had crumbled to 38/58. Democrats had come to oppose the idea overwhelmingly, 13/86. That’s the “Trump effect” at work.

You see the same thing happen on the issue of mass deportations, although the effect is less pronounced there because most Americans opposed that idea to begin with. Here’s the partisan breakdown the last time CNN asked whether the U.S. should try to deport all illegals, back in December:

And today:

The biggest movement, interestingly, is among Republicans. Maybe some righties put off by Trump have come to rethink their immigration stances as well.

The share of voters who say that mass deportation should be either the top priority or the second priority for U.S. lawmakers vis-a-vis legalizing illegals or preventing them from entering the country has also declined from September, from 36 percent at the time to 30 percent. Once again it’s Democrats who are driving that, dipping from 26 percent to 16 percent now, but indies and GOPers have seen declines too. And as for the all-important question of whether voters believe it’s likely that President Trump will succeed in getting Mexico to pay for the wall if it’s ever built, well…

Republicans are narrowly split but even they lean towards disbelief. The deeper question in all of this if you’re a border hawk generally and a fan of the wall in particular is whether the issue can or will ever again be decoupled from Trump in the public’s mind. If he loses this fall, maybe in due time stronger borders won’t mean “Trumpism” to as many Dems and independents and some of the democratic leverage to pressure Congress into better enforcement will be regained. Trump is a vivid character, though, and his alt-right fan base may have staying power within the GOP even if he doesn’t. If border enforcement comes to be seen by more people as a hobbyhorse of “white identity politics” and/or white nationalists, the lopsided Democratic opposition will entrench and more independents may peel off. Just as Trump fumbling a winnable election to Hillary would cinch mass amnesty next year, Trump making this issue his own may seal its doom long-term.