Don't recall who wrote this, but it's a fact that at one point in his career, Trump performed as a professional wrestler on TV. In other words, he took part in a sport that most fans realize is fake, but that they enjoy anyway because it's entertaining. Also, pro wrestling fans tend to like wrestlers who pretend to stand for their own values -- the populist hero wrestler against the cartoon banker wrestler, the 100% all-American hero wrestler against another wrestler pretending to be a jihadist terrorist, etc.
What's probably true about many of Trump's supporters is that they never believed he would literally force Mexico to pay for the wall. They knew at some level that this was a fake promise, but they loved it anyway because it reflected their values & wishes and because it was entertaining.
From this perspective, Trump's promise to make Mexico pay for the wall was always Reality TV, something he said for its entertainment value, even though almost everybody realized it was fake.
Now Trump wants to force the Democrats to let the taxpayers pay for the wall, again partly for the entertainment value. It makes for good television. And this Reality TV show featuring the Democrats is just as good for Trump's purpose as the old Reality TV show about making Mexico come up with the money.
-- democratic socialist