Is it time to go negative?

Hillary has pledged to largely stick to the issues and stay above the fray. However, in Michigan, Sanders hit Clinton hard on a variety of subjects. I'm not one to go negative in politics, but if you scratch the surface about Bernie, there is a gold mine of huge concerns. This fact is one reason why I support Clinton because Sanders' biography is so controversial (kryptonite) the result would produce a Republican landslide with many down ballot losses for the Dems, and of course the loss of the Supreme Court, Obamacare and all that would mean. His ties with Marxists and Communists (his videotaped praise of Fidel Castro during the Cold War, etc.) are particularly troubling. Overall, a Sanders candidacy would set back the liberal cause for many years.

The other troublesome point is that Sanders has no plan for implementing his platform other than raising taxes. He likes big ideas but ask him specifically how he will implement any of this, and there's nothing there. I also have an ethical problem with Sanders. He's been in Congress 25 years - long enough to know that his platform doesn't stand a rat's chance of getting passed by Congress because of simple math (an historic 58 seat Republican majority in the House and a substantial one in the Senate) and the rabid House Republicans. Either Sanders is conning his supporters or he honestly believes that it will pass. I don't know which is worse about the guy.

So my question is: Should Clinton go after him? Or continue with her approach?