So with that caveat in mind, let's look at a poll from Suffolk University and USA Today that was released on Monday. The numbers suggest that large chunks of both parties would be inclined to vote for the other side or a third party if their preferred candidate doesn't win the nomination.
Only about 6 in 10 supporters of Trump, John Kasich, Ted Cruz and Bernie Sanders say they'd support another candidate from the same party if their guy doesn't get the nomination. (Incidentally, if Trump lost the nomination but ran as an independent, two-thirds of his supporters would go with him.) Only supporters of Hillary Clinton stand out; 8 in 10 of her supporters say they'd be team players if Sanders won.
Why those margins? Probably because Clinton supporters think she's going to win anyway. Suffolk asked everyone who they figured would ultimately prevail in the general election. A majority of Clinton, Sanders and Kasich supporters figured it would be Clinton.
It's much easier to say you'd back your opponent if you have little concern that your opponent will actually win. If you'd asked the Yankees at the start of Game 4 of the 2004 ALCS if they'd root for the Red Sox in the World Series if the Sox got there, the Yankees would have laughed and said sure.
We can point to another, more applicable example of that. In 2008, Gallup asked a similar question at the height of the contested Democratic race that year. At the end of March, Barack Obama had already built up a large lead over Hillary Clinton. His supporters were much more likely to say they'd back her than vice versa. By the time the primaries were over, a contingent of Clinton fans pledged not to back Obama. But that November, far more of them supported Obama than it seemed would in the March polling.
Contested primaries are, by definition, contentious. That provides a natural disinclination to view opponents charitably. The question is whether or not supporters of losing candidates will end up backing the other candidate from the same party. Or if, once again, 2016 will prove to be exceptional.
We know better by now than to try and predict that.