"Mrs. Clinton may find it not so easy to repeat Mr. Obama’s gains. There has been weakening in her support among Mr. Sanders’s supporters over the last month or so. In April, Mrs. Clinton held between 71 and 82 percent of Mr. Sanders’s supporters; today it’s between 55 and 72 percent.

Exactly what’s driving the shift is hard to say. What’s clear is that Mrs. Clinton’s challenge isn’t totally superficial. Just 20 percent of Mr. Sanders’s supporters have a favorable view of Mrs. Clinton in the most recent New York Times/CBS News survey, while 47 percent have an unfavorable one."

http://www.nytimes.com/2016/05/25/upshot/explaining-hillary-clintons-lost-ground-in-the-polls.html?_r=0