Nate Silver, the Democratic primaries, and the trouble with Likert scores

Yesterday, Nate Silver tweeted out a table purporting to calculate Likert scores for a list of potential contenders in the Democratic primaries:

This ranking, he added, “isn’t that far from how I’d rate the candidates chances.” But is this actually a plausible way of assessing a candidate’s chances? And did he even calculate the Likert score correctly? S…