Agreed. It is impossible to be a Christian and a Democrat. You’re either one or the other—not both.
While I agree with your premise, I know Democrat Christians who will focus on the Democrats “concern” for the poor as evidence of Christian influence. Some Democrats have even convinced themselves they are better Christians than Republicans because of that and Democrats’ stewardship of the earth (global warming) and civil rights for all (tolerance for all). Sadly, many, many believe this is being Christian while hard-hearted conservatives can’t possibly be true Christians. Selection perception works well on cognitive dissonance.