Gramm might have been a good president but he was a TERRIBLE candidate. As you say, I don’t remember much animus on their conception of a flat tax, only fine point discussion.
Dole only went with the flat tax when he was desperate at the end, as I recall, so it wasn’t so much that he was anti-Forbes as he took up the mantle late in the game.
But you’re right—having been reminded, now I DO recall Bush bashing him on it. I still forget what Bush had to complain about, but I remember he had something he disliked about it and he smacked Forbes on it at some point.
FWIW, I'm not sure if there was even any kind of significant difference (I think Graham kept more deductions).