Sad to say this is a real possibility.
How ironic it would be to have the THIRD Clinton plurality win for POTUS.
My bet is that the GOP establishment will win its primary war against the conservative base and nominate Jeb Bush or someone else of that ilk. In the general election conservatives will either skip the top of the ticket, vote 3rd party, or stay home entirely.
But if there is a warp in the time-space continuum and the Democrats nominate James Webb some conservatives will end up voting for him.
A more likely scenario is Obama declaring the 22nd Amendment null and void, running for a 3rd term, and then announcing that he is Emperor for Life.