Au contraire. If Obama is denied the nomination [even if this tape exists], the black vote, the Dems most loyal constituency, will be devastated. They will not only stay home in much greater numbers in this election, but for many more to come, or they will move elsewhere. The Dems are between a Barack and a hard place. In some respects, it would be better to lose this election with Barack then face the long term consequences if he is replaced by Hillary.
This is why the Repulicans would leak this tape to a news source now, right after the primaries are finished and before the convention. If they played it in the fall, it would look bad after how much animosity exists between Obama and Clinton supporters. They know the SD’s will line up for Obama. Clinton supporters will think the Democratic party has been hijacked by the lunatic fringe. This will ensure Clinton supporters will have every reason to vote Republican in the fall. Change of plan and possibly too many people have seen it.