Barone sees black Americans as a declining percentage of our population, although there is no reason to believe they'll be anything but Democrats. Hispanics are not a unified bloc, a non-trivial percentage vote Republican, and they no longer threaten to become the majority of our populace.
Finally, in the last few years Asians are the most rapidly growing minority and are quite diverse in point of origin. They are also the least monolithically Democratic group of the three. You'll like his conclusion:
Romney can win even if 80 percent of nonwhites vote again for Obama. And rising percentages of nonwhites in future electorates will pose challenges, but not threaten doom, for the Republican Party.