I know this is off the wall, but...
Let's say there is a double dip recession and unemployment teeters around 13-14%. Afghanistan sees a rising death count with a majority of Americans favoring an exit, and Obama's approvals are at 37% on election day.
After an arduous struggle in the initial few primaries, New Mexico Governor Gary Johnson capitalizes on a divided field, winning decisively during the March 6th primaries after already building up momentum, and clinches the nomination. To shore up conservative support, he selects Senator Marco Rubio of Florida.
With evangelicals still not placated, Arkansas Governor Mike Huckabee launches a third party campaign with the support of big donors such as James Dobson, etc. He selects the Governor Nikki Haley of South Carolina.
So that being said, the tickets are...
Barack Obama/Joe Biden
vs
Gary Johnson/Marco Rubio
vs
Mike Huckabee/Nikki Haley