It was brilliant how candidate Trump added that issue on top of the border wall issue to raise the stakes and control the narrative.
Instead of declaring the wall impossible to build and/or too costly, all of a sudden the original concept was accepted when Trump baited them into changing their narrative to "Impossible! Mexico will never pay for it." It's as if he was haggling and his counter-offer was higher than the original price.
In the 2020 election cycle, he can point out all the miles of wall he has built, and when he is called out on the lack of Mexico payment he can say, "Oh well."
Quite the slight of hand by The Great Trumpini.
THE GREAT TRUMPINI!
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LUV IT