After 9-11 all it would have taken would have been an announcement from the atty general that the borders must be closed to prevent the infiltration of terrorists. Anyone found crossing the border would be fired upon as a suspected terrorist infiltrator.
In a month, no one, nadie, would be crossing the border. The word would get around fast that Uncle Sam is serious. Nobody outruns a 5.56mm.
The problem is, Uncle Sam isn't serious.
bingo ! Would have had close to 100% support at the time.