Actually, I don't think this is true.
The Bush DUI had been "common knowledge" for years. Yet the MSM used it as an Oct. surprise to get uninformed conservatives to not vote for him.
Same would hold for Obama. Cocaine is worse than marijuana or alcohol. There are a lot of uninformed social conservatives/fiscal liberals voting for Obama (a lot of Catholics for examples) who would turn off on Obama if they found out he admitted in his book to cocaine use.
Good point! I didn’t think of it that way!