So what’s going on? Sanders is maxing out on gains simply because of increased name recognition. Different pollsters ask about favorability and name recognition in different ways — making comparisons tricky — but the University of New Hampshire (UNH) polled Democrats in the state in April, June and July. Sanders’s favorable rating went from 45 percent in April to 66 percent in June and then to 69 percent in July. The share of respondents with a neutral opinion or no opinion of Sanders fell from 44 percent to 24 percent and then to 20 percent during that period. In other words, between April and June, Sanders was picking up low hanging fruit: The liberal wing of the Democratic Party learned about Sanders and liked him. But now, most voters who are predisposed to like Sanders already know about him.

This phenomenon can be seen when we compare Sanders’s current position to where Sanders and Massachusetts Sen. Elizabeth Warren were polling in April. As I wrote when he got into the race, Sanders had the potential to pick up a lot of Warren supporters; the two have nearly identical voting records in the Senate. Their supporters can be defined as the anti-Clinton left. The combined vote percentage for Sanders and Warren in the April UNH survey was 33 percent — just about the level of support that Sanders alone had in the July UNH poll. In other words, Sanders has won over the liberal flank of the Democratic Party and hasn’t grown much beyond it.