Trump critics were told over and over again during the campaign that we took him “literally but not seriously.” By that, his apologists meant that his campaign rhetoric should be taken as metaphor for the grievances expressed by the members of his base, not as actual policy prescriptions.
The wall is a great example of that. Surveys done during the campaign indicated that Trump supporters did not really believe he could force Mexico to pay for it. Promises about the wall were simply viewed as expressions of solidarity with those who felt that outsiders–Mexicans, Muslims, African-Americans, whatever–were ruining our country.
The irony is that it is clear now that Trump actually feels bound by the campaign promise even though his supporters won’t hold him to it. It can’t get much more ludicrous than that.