Notice that Bill Clinton was almost at 50% in 1992 but won in a three-way race. If someone other than Trump is the Republican nominee, he may play the Ross Perot role in 2016. If Trump is the nominee, and looks likely to beat Hillary or whoever the Democrat is, then we could find another candidate put forward to split the anti-Democrat vote again.
Anyway, Obama has signaled that he doesn't intend to let the Democrats lose this election...the only question is how he will use the powers at his disposal to accomplish that.