The Dims only chance of winning (barring an October Surprise crippling the Republican candidate) is to find a total unknown late in the Primary cycle like they did with Carter.
All of the present front runners are well known to the public as loons and idiots.
They also did pretty much the same with Clinton, he was unknown ot most people at the time he won the nomination. I guess being unknown and a convincing liar is their ideal candidate.