Each of the posters above have good assumptions and opinions. No doubt, but my opinions are not the sole reason I base my pick of Dempsey to kp Marciano... I constantly have read boxing history since I was a boy and I place much of my reasons to pick Dempsey, because of what hard nosed boxing writers who saw them both at their bests and almost unanimously chose Jack Dempsey to stop Rocky because of their respective styles as Dempsey was as mentioned a faster, shorter and accurate puncher than Marciano who missed half his punches. Against a fast murderous punching Dempsey and Joe Louis that would be a recipe for disaster....I also think that a young Mike Tyson would be a much greater threat to Dempsey because of his great power thrown with great speed and accuracy. Styles, styles, styles...
I do see Rocky winning the championship rounds big, provided he gets their of course. He might even get Demspey out of their if he's well behind on points and throws serious caution to the wind.
People are listing the advantages that all the best fighters Marciano faced had over him... And still, 49-0. He wasn't exactly shocked or out of his element when he got in against a faster or more accurate opponent. He was a supreme operator. He had titanic reserves of stamina and was almost never untracked from his target. I don't think Dempsey could hold him off long enough before he succumbed to what was a relentless, unending flood of powerpunches. Look at Moore's expression while on the canvas. A man who had fought everyone and KO'd most. As skilled a boxer-puncher as the sport really has known. He was out of answers. Best to stay on the canvas.
Moore, Walcott nor Charles hit like Dempsey. Not close. Add to this the great difference in skill level. Dempsey the far better boxer/better defensively. Dempsey was also much quicker. The deck is stacked very heavily in Dempseys favor. There is not a single category where Rocky has an advantage. One could say conditioning but prime Dempsey was well trained to fight 15 hard rounds. Not an ounce of fat on prime Dempsey.
That's a titillating description of Dempsey's physique. Kudos. However, Moore is the most prolific KO artist in the sport's history. To say he is not in Dempsey's league is either disingenuous or pure fantasy.
As a LT hwt not a hwt. Sharkey stated Dempsey hit harder than Joe Louis. That puts Dempsey at the highest echelon of heavyweight power punching. Moore was not in that league. Sharkey further stated that "he never thought anyone could hit that hard" regarding Dempseys power. Please name another former hwt champion that made the same statement regarding Moores punching power.
Sharkey was mentally unstable. Dempsey weighed 180 or less for his best and most notable performance. Was Moore really smaller? And by the way, Gibbons said that Miske punched harder.
OK let me dip my toes in briefly, for real. On paper, Dempsey has all the required tools to win against Rocky. He could theoretically use his speed and accuracy to pick Marciano apart and wait for the counter. In reality, he comes to the ring, sees Marciano and thinks "da fuk is this little shrimp in my ring? I'll clean him up quickly and go bang a starlet". And then he finds out who Marciano is. Short version: Dempsey liked to brawl and brawling with Marciano is gonna get you KO'd. I don't think he would be able to help himself.