The purpose of the illegal caravan is not to vote, it is to tie up immigration control personnel. They can’t be in two places at once.
A lot of it depends on what the caravan does when it arrives. If it splinters/fragments, then its just a statistical blip over the situation we have now. A few extra people trying to sneak in via the usual routes. If they come en mass to one or a couple of major crossings, then a) do they go through the process single-file - which means they are fingerprinted, interviewed for asylum, detained or b) do they riot and try to break down the gates? Different responses, and I am sure Trump will prepare for either eventuality. He’s politically tied to it, and experienced enough to know to order the use of the least lethal methods available to repel.
I am no military tactician, but tactics for repelling an invasion force goes back to biblical days. I’m not all that worried about it, even if the media plays it up big. I just don’t want to see bloodshed over it.