Jim Wallis brought his ‘Jesus would be a Democrat’ road show to a Presbyterian (PC USA) church in my area last year. Fairly appropriate, since most of the Protestant churches where I live are basically doing the business of the Democratic party now. Whenever I see a bumper sticker like the one you mentioned, I always want to note to the person some true famous liberals and socialists...Joe Stalin, Pol Pot, Fidel Castro, Mao, Ho Chi Minh, Adolf Hitler....
Liberals are about making themselves feel better. They don't want to hear what their "solutions" do (like you say - Hitler, et. al.)