https://www.cnn.com/2019/03/20/politics/bernie-sanders-favorable-unfavorable-rating/

“Return to the mean” is just terrible. Here, it’s taking the shine off a rock star, and the same might happen to O’Rourke and Biden.

«A CNN poll taken in early December 2018 gave him a +13 net favorability rating with all voters. A Gallup poll in September 2018 had him at a +15 net favorability rating with all adults.

So, what’s changed? I’d argue that Sanders was benefiting from not being in a competitive campaign. (Former Vice President Joe Biden, who has garnered the most support in general election polls among the Democrats, may be benefiting from a similar effect.) When you’re not being thought of a viable threat to win a party’s nomination, opponents tend to lay off. The last time Sanders was thought of as at least a minor threat to win the Democratic nomination was in March 2016. His net favorability rating back then among all voters was +3 points in a CNN poll.»