That's not how politics works, and the fact that Trump is generally capped at 35% of the vote in the republican primary, and Sanders is losing the democrat primary despite attacks, is demonstration that what you're saying is wrong.
Seriously, if you don't think Sanders negatives will go up if he is actually attacked for once I don't know what to say, other than it would be good to read up on past USA elections. Negative campaigning works - Obama did it, Bush did it, Clinton did it. IT's part and percale of the landscape, and so far Bernie hasn't suffered that scrutiny.
(In one way Clinton's unfavourables aren't as bad as they look because it's all baked in - at this stage there's basically nothing that can be done to lower them further because she's been the target of rabid right wing attacks for 25 years now. Yes they aren't great, but they won't get any worse - whilst Sanders absolutely will, as will any republican nominee not called Trump)