I guess my definition of mandate is based on numerical dominance of the vote -- I'd expect a 60-40 margin in the popular vote before I'd begin to think mandate, and probably 75-25 in the electoral college. But that is just my opinion.
Think of it as a dinner party with 20 guests that split 11-9 in their opinion. That is a 55-45 split. One person in that group changing makes it a dead heat. Therein lies my definition of "mandate" and my belief that we do not have one -- and haven't for quite some time.