Well, I suppose it's possible, but it's still very unlikely. To understand what's happening with the polls, you have to look at what stage the nomination race on each side has reached. All Trump's rivals for the Republican nomination have dropped out, leaving him unopposed. Faster than anyone (including me) expected, the "NeverTrump" movement among Republicans is imploding and supporters of other candidates are rallying around their now-inevitable nominee. The most recent poll I could find (the Rasmussen one) has Trump getting 76% of the Republican vote, and I've seen figures as high as 87% in other polls. This is what normally happens. During primaries, voters often say (and believe) "if X is the nominee, I'll never vote for him in the general", but once the nominee is chosen, party members rally round. There had been much speculation that this normal pattern would not hold for a nominee as bizarre and repulsive as Trump -- all we're seeing now is that this speculation was wrong, or mostly wrong.
On our side, the race has not yet reached that point. Hillary's nomination is pretty much inevitable, but that's not obvious to people who aren't following the process with political-junkie dedication. Bernie is still very much in the mix and making plenty of noise. Not until Hillary has clearly won -- perhaps not until her official nomination at the convention -- will the same rally-round effect that is now boosting Trump's numbers start to do the same for her. (There are a few other reasons why polls may be over-favoring Trump.)
Will Bernie's supporters eventually vote for Hillary in the general? While some of the rhetoric we've been hearing is disconcerting, I think most of them eventually will. As precedent, I would cite the 2008 election. I remember the fury and disappointment of Hillary's supporters very well -- because I was one of them. I remember the PUMA movement. But in the end, most did vote for Obama against McCain. I think Bernie's supporters will follow the same pattern this time. Some may vote for Trump or not vote at all, but not enough to make a difference. Trump is, after all, a vastly more repugnant and frightening figure than McCain was. And Bernie himself will probably support Hillary once she's nominated, however grudgingly, especially if the party gives his agenda some substantive concessions (he's earned them). He doesn't want to go down in history as the man who helped elect Trump.
In the quiet privacy of the voting booth, I suspect that even some of the saner Republicans will have second thoughts about whether a man of Trump's temperament and unpredictability should be put in charge of the world's most powerful military forces and 7,000 nuclear weapons. As Comrade Misfit pointed out this week, the most memorable attack ad in history is almost perfectly suited to use against Trump:
In the meantime, there are actually some positives to the fact that Republicans are mostly rallying around Trump. I had hoped to see a divided right wing giving Hillary a massive Electoral-College landslide, and then the Republican party tearing itself apart in recriminations -- but if that isn't going to happen, consider the implications. Hillary will still win. Trump has said more than enough to expose his true nature and intentions and to make it clear that we Americans, this year, are being weighed in the balance of history much as the Germans of 1933 were. So, yes. Let the Republican party and conservatism as a whole openly and publicly commit themselves to the wrong side in this momentous choice. And let all of them, forever, bear the indelible stain and disgrace of their decision.