In explaining his decision to run for President, Trump described himself as Hilary's worst nightmare. Trump hasn't been reading the news. According to CNN, Bernie Sanders would beat Trump by an even larger margin than Hillary Clinton.
Hillary's worst nightmare is a real liberal, capable of getting liberal votes instead of merely getting "Defeat the Republicans" votes.
To emphasize: Bernie Sanders had better polling against Trump than Hillary Clinton. Among registered voters surveyed for the CNN poll, Clinton would beat Trump 56% to 40%, whereas Sanders would defeat Trump 59% to 38%.
As for Trump, his "I'm not Hillary Clinton" candidacy doesn't seem to have done him much good among voters, even if he's got the highest name recognition in the un-thinned Republican field.