I think the argument that Bernie supporters will not vote for Clinton if she is the eventual nominee appears quickly to be an unsound one if you have any familiarity with American politics.
Consider possible general election contests under this category:
Clinton v Trump
Clinton v Cruz*
Clinton v Rubio*
Bernie's support is highly concentrated among young, white, and educated voters (http://fivethirtyeight.com/features/why-young-democrats-love-bernie-sanders/). This voting demographic typically self identifies as very liberal, and in all three of these contests Clinton is obviously the choice more in line with their views. Bernie's attacks on Trump have made some of the most popular speeches so far in the Sanders campaign. Moreover I'd expect Bernie to endorse and speak in support of Clinton if he loses the primary contest.
Its likely these demographics will have a lower turnout in support of Hillary relative than what they would do for Bernie, but its also highly improbable that anything more than a miniscule percentage of these voters would defect to trump.
Thinking of it as Democrats and Republicans really oversimplifies the issue and if you live in America and pay attention to politics this is pretty obvious to see.
*Of course Trump is favored now and I admit seems to be the probable nominee, but the other two candidates aren't utterly out of the picture yet.