^^^ Yes. She still has two years left in her term, so she doesn't have to step down.

Obama also has two years left in his term. So does McCain. Both return to their seats if defeated.

Joe Biden's term expires in January. However, Delaware and a handful of other states (Texas and Connecticut, maybe others) allow a person to run for two offices simultaneously provided that one of those offices is President or Vice President. I believe that Texas was the first state to allow this, in 1960, for LBJ.