The migrants aren't going to be able to influence the midterms per se. The plan is to create a media clusterfuck at the border right at election time. Trump is put in a lose-lose situation. If he allows them in he looks weak. If he stops them with the military he looks like a cruel dictator.
Fortunately, he outsmarted them by having Mexico stop the caravan at their border. Now with the footage of the Honduran horde storming the fences, money being passed out, etc. He's won again.
Deepstate outmaneuvered once again.