@sozobe,
I am worried more that Sanders' supporters will just stay home in November than that they will vote Trump. The Democrats need to take control of the Senate more than anything else, so that means they need a good turnout. That is not just for the Supremes, either, as important as those nominations are. The White House nominates the entire Federal bench, and of course, all of the executive branch offices, including offices people don't usually think of such as the Office of the United States Trade Representative.
The 2008 situation was more like 1980. In 1980, Reagan was the outsider, the dark horse, and George Bush was the establishment candidate. In both 2008 and 1980, that outsider won the nomination. This situation is not the same. I'm not really worried about Sanders supporters voting for Trump so much as i am that they just won't vote at all.