There are a lot of articles around which say that Democrats should not assume that Trump, if he is the nominee, will be an easy defeat in November. Personally, I think he will be but that the Dems shouldn't presume that and should run as if he's a real threat. For what it's worth, Daniel Larison over at the American Conservative thinks Trump would not only lose, he'd lose big enough to damage the Republican brand in many ways.
I keep saying that I will be convinced that Trump might win when someone can show me numbers that suggest he will flip enough states Obama won to get an electoral advantage. As Josh Marshall notes, if he can do that with any large state, he oughta be able to do it with Pennsylvania…and the polls there currently show Hillary beating Donald by double-digits. Yeah, he's sewing up the disenfranchised white male vote all across America but so did Mitt Romney.
Of course, there's a long way to go and already a lot of things that no one thought could happen have happened. I just don't see much chance yet of Trump picking up the non-white or female vote, or for millions of angry white guys who haven't voted in the past to suddenly materialize at the polls. What I'm afraid of is not that Trump has the momentum to win but that suddenly something will come along — some new revelation or actual evidence of wrongdoing — and there will be a much better reason than there's ever been to not vote for Hillary. That's assuming it's Hillary.