The Democratic Party might as well be the official party of Black America. Therefore, anyone who runs against a Democrat is, ipso facto, a racist, opposed to the interests of Black America.
It is also effectively the official party of American Jewry. Therefore, anyone running against a Democrat is, by definition, an anti-Semite.
It also bids fair to be the official party of the LGBT community, making anyone running against a Democrat a homophobe. See how easy this line of reasoning is? It's fun, anyone can play.
Or, turn it around. Historically, Republican voters were those for whom the society "works," its winners. Thus, if you run against a Republican you advocate failure to thrive, you represent losers, the not-coping, deadbeats.
The GOP normally gets the votes of most married people. If you run against a Republican you must support infanticide, broken homes, parentless children, blended families, and welfare dependency.
To run against a Republican, you must defend the indefensible, support the insupportable, and love the unlovable.