Short Version: Romney used the imagery of "hanging" to describe Obama's campaign in relation to a Different White Guy's campaign. America has an ugly, storied history of hanging black guys for no reason.
Less-Short Version: It seems so long ago now, but in 2011, candidate Romney made a gaffe while speaking about President Obama that was likely cluelessness on his part. Clueless or not, his campaign still rushed to clarify Romney’s meaning after he said the following during a New Hampshire campaign stop: “Reagan came up with this great thing about the ‘misery index’ and he hung that around Jimmy Carter’s neck and that had a lot to do with Jimmy Carter losing [...] Well, we’re going to have to hang the ‘Obama Misery Index’ around his neck.” Romney quickly threw in the word “metaphorically” for a save. Smooth, Mitt. Smooth.