The Democrats were actually able, with their aggressive campaigning, to talk Republicans out of voting against Obama!
Now you have seen how it is done; Convince one side their candidate is great or convincing the other theirs is a poor candidate, or hopefully both, and one will sit out the election. Mostly, that will be the Candidates responsibility.
In our lifetime, few will ever be coaxed away from their political party. It will, instead be a matter of who votes and who doesn't.