Bernie is an enigmatic candidate who tends to reel in the voters that are most hard to sway. I can see this as the only way for him to really hurt Trump's campaign. The media and other candidates have been hacking away at Trump for months with little to no effect, and I don't imagine Bernie would be any different.
While there was a lot of concern over the past months that Sanders might negatively impact Clinton's campaign, it now seems like the Democrats are uniting to attempt to defeat Trump. If Sanders can sway many of his supporters into supporting Clinton, this could help create a powerful coalition to defeat Trump.
Although he has a fairly large fan base, Bernie supporters are shown in the polls to have a strong disliking for Hillary Clinton. Therefore, his supporters have the choice to vote for Trump, stay home, or vote for the libertarian candidate. Recent polls and interviews also suggest that many Berney supporters will vote for Trump. I think this is probably because of the 'political outsider' attribute that they both possess.
No, Bernie Sanders will not negatively impact Trump's campaign. By not choosing to withdraw from the democratic contest after Hillary Clinton has clinched the nomination, all Sanders can do is divide the party and pull votes away from Clinton. This allows Trump, who is now uncontested, to focus on gaining the votes of those on the fence, while Hillary still has to deal with whatever Bernie says about her.