If this really happens, and Hillary is the nominee, she will be badly damaged, and there will be a lot of resentment on the part of the Obama supporters, many of whom will suspect (perhaps rightly) that Hillary was behind the release of the tape.
If the GOP really has it, why would they release it now? Answer: they wouldn’t.
Hypothetically? Becuase NOW it inflicts maximum dmage on BOTH DEM candidates. It tainst Hillary! with suspicion of being behind it, and it taints Obama for being married to a closet racist.