Bernie Sanders newest commercial.

Currently Sanders is doing surprisingly well in the polls: 

Vermont Sen. Bernie Sanders' favorable rating among Americans has doubled since Gallup's initial reading in March, rising to 24% from 12% as he has become better known. Hillary Clinton's rating has slipped to 43% from 48% in April. At the same time, Clinton's unfavorable rating increased to 46%, tilting her image negative and producing her worst net favorable score since December 2007. 

Sanders' increased favorability reflects the broader increase in the public's familiarity with him since March. Overall, 44% of Americans are able to rate him today, up from 24% in March. Not only has the percentage viewing him favorably increased, but also the percentage viewing him unfavorably has risen, up eight percentage points to 20%.

It should also be noted that some of the conservative presidential candidates, such as Ted Cruz, are bending over backwards to praise Sanders in the hopes that he might beat out Hillary for the nomination.

However if I were a Republican candidate I might not be so convinced that Sanders represents the weaker candidate.