Harry Enten at Nate Silver's FiveThirtyEight blog, reports three recent polls which show that roughly 1/3 of Sanders backers cannot support Clinton. They won't necessarily vote for Trump but Clinton will miss the basically white Millennial votes they mostly represent.
The polls ask for preferences with and without third party candidates shown. Clinton loses 12% when third party candidates are an option. Those are likely former Sanders voters who will have third party choices come November.
