Thanks for Clinton folks, the only person she can beat is Cruz or Trump, good thing the other side is dumber than we are.
Sanders hasn't been tested on a national level. Before he ran, almost nobody outside Vermont had ever heard of him. Since then, more people know his name, but he has been free of attacks from either Clinton or the Republicans, who have not missed an opportunity to trash Hillary in the debates (or elsewhere) and have pretty much left Sanders alone, since they don't consider him a threat. Sanders has also been very critical of Hillary, even though he pledged last year to run a clean campaign. So, Hillary has been constantly under attack by the Republicans for at least a quarter-century, and this year she has been the target of attacks from either side. It's amazing that she's still polling well and leading the Democratic primaries.
Sanders, on the other hand, has benefited from no mud-slinging so far. If he were to get the nomination, his favorable ratings would plummet, because the Republicans would not hold back. They will try every dirty trick in the book, and Sanders would be an easy target to take down. For one, he calls himself a "democratic socialist" and has extolled Communist leaders in the past. (In his tenure, Obama has been accused of being a socialist and socialism has become a dirty word to many Americans.) They will also bring up that he didn't hold a steady job until he was around 40 and didn't accomplish much in the House and Senate. Remember, in 2004, the Republicans managed to discredit Kerry's military record and attacked his subsequent antiwar activities (i.e. swiftboard) despite the fact that Kerry actually served in Vietnam and Bush didn't.
In short, Sanders is untarnished... for now, which is why he's polling so well. But it won't remain so if he were to be the Democratic nominee and the Republicans took out the knives and daggers.
Salve, Regina, Mater misericordiae
Vita, dulcedo, et spes nostra
Salve, Salve Regina
Ad te clamamus exsules filii Eva
Ad te suspiramus, gementes et flentes
O clemens O pia