There is no way that the Dems can throw Obama under the bus at this stage no matter what his wife might say on the tape, if it exists. Blacks are their most loyal constituency. "Stealing" the nomination from the black guy will have far reaching consequences not only for this election but for many more to come. The Dems want to be the home party for all minorities who will be the majority by 2050. Far better to lose this election than jeopardze their future prospects.
For now, the Dems are between a Barack and a hard place.