Exactly. Trump has gone after his GOP opponents and Billy Bob, but has been rather tame in his attacks on Hillary.
I actually think this kind of attack works, and makes sense early in the general election campaign. Bill still has some cache with some groups, but he's not nearly as popular with others -- particularly younger women. These attacks are designed to make her choose to which group she is going to pander, and because they're not directly on her, it makes them a difficult thing to counter.
I'll give his campaign some strategic credit for realizing that the image of Bill as a great President in the minds of many independent voters didn't apply to millennials, and also didn't take into account some changed mores. So his campaign decided to make the "first impression" for millennials about Bill, and it seems to be working. It's certainly not a "killing blow", but more of a "preparing the battlefield" attack that will limit her options in the general election. And at the same time, it keeps Trump on the offensive without blowing all the anti-Hillary ammunition they've saved up.
