Yes, 1988 is ancient history by US standards, but let's have a look anyway. Here is the ad for those unfamiliar with it:
I have heard many times from progs that Dukakis lost because of this ad. Specifically, they say that he looks silly in that helmet, even though it only appeared for a moment at the end of the ad.
But isn't the content compelling? The majority of the ad is just text explaining how he opposed various military projects and then at the last moment shows him posing with a tank. The contrast between his actions and his posing is powerful. Keep in mind too that the US at the time had been locked in a long struggle with a powerful enemy.
This ad didn't make Dukakis look bad because of the helmet. It made him look bad because it showed that his actions didn't match the image he tried to put out. There is powerful delusion among progs to attribute their failures to anything except their own ideas.