A toughie. Clearly for Robertson to win the zeitgeist would have to be even more right-wing than it was IRL in 1988. It seems certain, though, that Robertson would turn off a lot of voters, Reagan would have a hard time campaigning actively for him (as he did for Bush IRL), and a major 3rd party movement would develop. Maybe something like this:
Dukakis/Bentsen 38% / 260 EVRobertson/Helms 37% / 240 EVIacocca/Perot 24% / 38 EVWith no EV majority, the election would go to the House, which would select Dukakis/Bentsen.