It's next to impossible to win a libel/slander suit in the US, and it's extra-impossible for a public figure to do so, such as Cain.
I know this is Apples & Oranges, but back in the early 90’s my company was sued three times (all frivolous AND NOTHING TO DO WITH SEXUAL HARRASSMENT). Each time I wanted to counter sue but my lawyer advised not to bother as such counter suits rarely are successful. Finally after being sued a 4th time I fired our attorney and hired a new one at almost 5 times the former’s hourly rate. The new attorney not only filed a counter suit but also later in the process sought sanctions on the opposing attorney's. That lawsuit disappeared rather quickly and we haven't been sued since.