Their factions have always had the self-discipline to rally around the nominee. Throw in the inevitable circle jerk of the convention when everybody makes nice, plus the campaign demonization of the Dem nominee as TEH GRAVEST THREAT TO OUR FREEDUMB EVAR!!11!, and they'll have no trouble keeping the ranks together for the election.
The real change in the last few cycles has been that the Dems have managed to do the same. They've somehow actually figured out how to be an effective political party again after a 20-year run as Charlie Brown. The question with the Dems is always, will that continue. I have a feeling Hillary will be able to herd those cats, through an appeal to sheer pragmatism. She's the anti-Obama -- he ran on "love me"; she's going to run on "hate me, but hate them more."
A shame she isn't a Republican. She missed her calling.