People held a sign during a protest called 'Wall Off Trump' near site of the Republican National Convention in Cleveland in July. (EPA/Justin Lane)

Donald Trump has said hundreds of times — including after meeting with Mexican President Enrique Peña Nieto a couple weeks back — that Mexico will pay for his proposed border wall.

Not even Trump's supporters believe that will happen.

A new poll from The Washington Post and ABC News shows just 21 percent of Americans think Trump will make good on his long-promised pledge to force Mexico to pay for a wall that is aimed at keeping its own residents out of the United States.

Fully three quarters — 76 percent — say he won't accomplish it.

Even a majority of Republicans think Trump will fail on one of his signature policy initiatives, with 52 percent saying he won't be able to get it done, compared with 42 percent who think he will. Among Americans who say they are voting for Trump in the poll, 48 percent say he'll succeed, and 44 percent say he'll fail.

Hispanics find the idea particularly preposterous. Fully 88 percent of them say Trump won't be able to do it; 10 percent say he'll get it done.

And this poll might actually be somewhat understating just how little Trump's supporters actually buy into the idea. A CNN-Opinion Research showed 25 percent said it was at least "somewhat likely" he would get Mexico to pay for the wall. But it also offered more than just two options, and it turns out just 9 percent thought it "very likely" that he would succeed — including just 18 percent of Republicans and 20 percent of Trump voters.

The proposal has been among the most talked-about Trump proposals of the campaign, in large part because it seems so dubious on its face. But as the media and opponents have largely laughed it off, Trump has doubled down over and over again. It even became a call-and-response at his rallies, with Trump saying, "Who's gong to pay for the wall?" and his supporters responding, "Mexico!"

But last month, Trump indicated he might be softening his immigration proposals, then met with Peña Nieto.

Trump said after the meeting that the two men hadn't discussed payment for the wall, which seemed to be a notable omission given how much the GOP nominee has been pressing the issue on the campaign trail. Peña Nieto said soon after that meeting that he did, in fact, tell Trump that Mexico wouldn't pay for the wall. (Trump later disagreed with that account.)

Hours after that meeting, Trump gave a fiery speech calling for massive increases in illegal immigration enforcement — and, of course, a wall paid for by Mexico.

"One hundred percent," Trump said. "They don't know it yet, but they're going to pay for it. And they're great people and great leaders, but they're going to pay for the wall."

Trump might want to tell that to his supporters — or at least, the ones who don't attend his rallies.