Donald Trump would be an embarrassment, a disaster, and an outright danger if elected President of the United States. This creates a joint obligation shared by all Americans to try and stop him from being elected. For Republicans, that's easily satisfied in the primaries (vote for another Republican) but very painful to do so in November. For liberals, there is an equally strong obligation to stop him from becoming President in November.
There is, however, no liberal obligation to stop him from being nominated. I have no intention of crossing over and voting in the GOP primary (even assuming the race is still undecided by the time my state rolls around).
Now don't get me wrong: I hope beyond hope that the Republican Party does not select Donald Trump as their nominee, though that hope is becoming increasingly dim. I hope that because even if it is true that Trump would be "easier to beat" than Rubio or Cruz, I still prefer that the forces of unhinged xenophobic racism be weaker rather than stronger, and a Trump nomination would indicate they are far stronger than previously anticipated. So I certainly prefer that Trump loses.
But the Republican nominee should be the person who reflects the preferences of Republican voters. One important function of democracy is aggregative -- it gives a rough sense of people's raw preferences. And it is valuable information to know that open racism, bigotry, anti-intellectualism, and thuggery have significant appeal across the base of a major political party. Again, I hope that it turns out that their appeal is not strong enough to carry a nomination. But already the persistent appeal of Trump has been forcing Republican intellectuals to grapple with facts about their rank-and-file that they've long been in denial about. Democratic cross-over voting would only serve to blur the truth and stall the day of reckoning.
Donald Trump has already forced Americans of all stripes to seriously reckon with a revived and rejuvenated racist movement in the United States. That effect would be magnified if he were the nominee, and it would accentuate the threat that this movement represents. But it still only a difference in degree, rather than kind, from what he's already accomplished. As President he could do far worse, and that is a terrifying prospect that should unite everyone. But the call to prevent Trump's nomination, as opposed to his election, seems to be justified based on little more than saving the Republican Party from additional embarrassment. And that isn't any duty Democrats are obliged to take upon themselves