Cleveland and JFK would be GOP'rs today. In fact they'd be Tea Partiers. It's sad many on the left think JFK would stand with them. The guy was a fiscal hawk who believed in sound currency (unlike the left of today).
JFK would definitely be a Democrat, although he'd be a moderate or conservative one, as he was in the 1960s. I agree with you that a liberal or left-wing JFK is wishful thinking on the part of today's progressives, but there's no way he'd be a Republican. I could see him as a Cuomo-Clinton type Democrat, though.
Only because his dad was a Democrat.