Probably the attacks would start with the rape-fantasy porn he wrote, move on the video of him at a rally with crowds chanting "death to America," maybe touch on Sanders violating campaign finance laws, and dwell for a while on his honeymoon in the USSR, and other things to freak out the olds, along with a lot of Castro connections to make sure Florida was ugly. The GOP had a big oppo research file on him, and a lot of it would have done some damage. None of it came up in the primary, but in the general there was a lot of dirty laundry that would have been aired. It was mostly crap that freak out people who aren't like me, but there are a lot of people who aren't like me.
With an election where the winning candidate had said, "women, you've gotta treat 'em like shit," and bragged about sexual assault on tape, god only knows what effect it really would have had on Sanders, though.