If, as Donald Trump's tweets portend, he will make an issue of Bill Clinton's affairs, at the 2nd Debate, it will certainly be beyond my comprehension, as to logic.
If the import of Trump's comments on Bill Clinton are that Bill was a good President despite being a predator, Hillary Clinton will be able to say: "Donald, I know Bill Clinton; Donald, you're no Bill Clinton."
If Trump tries to impugn Hillary's character because of what Bill did, she can just say that Bill's name is not on the ballot and Trump is free to vote against Bill if he chooses. She can also state she didn't approve of Bill's behavior any more than she does his.
If Trump says that she, Hillary, acted improperly towards Bill's "victims" she can assure Trump that she promises not to act badly towards Trump's victims, besides, "tomorrow morning, the fact-checkers may be advising you, once again, that the allegation is false".
Perhaps, someone will warn Trump that the pot takes its chances when calling the kettle black.