Pundits seem to have a problem getting their heads around this.
Donald Trump is saying that he will make Bill Clinton's past infidelities and misbehavior a campaign issue ("fair game") against Hillary Clinton. At the risk of stating the obvious, this is a tactic that may work great for Trump in a Republican primary - particularly with the people who make up Trump's core constituency. But in a general election, with an electorate not driven by the things that drive Trump supporters, having a thrice married, philandering blowhard like Trump trying to beat up on a woman over her husband's philandering, about which she is if anything the victim rather than the perpetrator, is almost comically self-destructive on Trump's part.
To put it in the sales language Trump seems most readily to understand, there's one product you can sell to agitated and conservative middle aged white men and quite another you can sell to ... well, everyone else.
I'm sure Hillary finds this grating and galling on a personal level. But she is if nothing else a realist. And in those terms, I'm sure she's thinking some version of "Please proceed, Governor."