Stay with me here...

The Democrat party founded the KKK.

Many of the leaders in the Dem party were high ranking members of the KKK (Senator Bird)

Dems are accustom to dealing with these racist across their leaders for decades.

Dems thus cannot envision a party that's not lead by racist.

Since Dems have very few friends who are Republicans, they don't know how Republicans really behave.

Dems presume Republicans are bad for being the 'other'

Dems presume Republicans are all racist since Dem leaders are racist.

Republicans being racist is a core belief of Dems

Not finding Republicans acting racist causes cognitive dissonance with their perception of reality

This cognitive dissonance is causing Dems to go crazy.