Writing at Townhall, Matt Vespa quotes NBC's Chuck Todd to the effect that 17% of Sanders voters plan to vote for Trump in November. He also quotes ABC's George Stephanopoulos claiming 15% of Obama voters will vote for Trump. There are video links at Vespa's site.
Both Todd and Stephanopoulos are effectively Democrat operatives with press credentials. Neither could have been happy saying what he said. Which, paradoxically, makes what they reported more likely to be true, perhaps even understated.
Sudden insight: Trump's appeal to restore American "greatness" resembles what Charles de Gaulle promised France following the debacle of World War II and Vichy. It worked for de Gaulle, got him elected, why not for Trump? Incidentally, the same sort of restorational fervor maintains Putin's popularity in Russia. Restoration is potent political "stuff."