The polls this year could be somewhat less predictive than usual, because both nominations remain contested. Some partisans who will eventually support their party’s nominee are working hard against that candidate right now, and are focused on all the reasons to oppose him or her. That is why we can expect the winners on both sides to get a boost once they win because the parties will at least partially unite behind them.
Normally the parties will rally around the winner. That will almost certainly be true on the Democratic side this year. As the Sanders campaign has signaled thoughout the campaign, he’ll be a good team player for the fall election.
On the Republican side, who knows? Trump can’t fully unite the party if he wins the nomination, and it’s possible he’ll prevent unity if he loses. And it isn’t clear that Cruz could unite the rest of the party, if he’s the winner, even without Trump throwing bombs from the sidelines. But expect at least some boost for the eventual nominee anyway. The incentives to unite are strong. There’s no way to know what the Republican Party will look like after its convention in Cleveland.