Frankly, I can't think of a single case since New York Times v. Sullivan in 1964 in which a public figure (like a presidential candidate) has ultimately won a verdict that was sustained on appeal.  That's because of the First Amendment - every defamation case involves the First Amendment.  For a recent case called a primer on New Jersey defamation law (which closely hews to federal law because of the First Amendment), see Trump v. O'Brien, decided September 7 of this year.  The Donald sued, saying he'd been libelled by a book that raised questions about his claims of wealth.  The author won on summary judgment.  The court felt obliged to note that evaluating the truth of the Donald's wealth - the author's job - was complicated by "... Trump's astonishing ability to prevaricate. . . ." and thus it as even more diffuclt than usual to say the author published falsehoods either intentionally or with reckless disregard for the truth.

Frankly, Cain hiring a prominent libel lawyer smacks more of a rich man using his money and influence to intimidate a poorer woman, already once his victim, by threatening further impoverishment.  Libel defense is difficult and expensive.