Allow me to set the stage for you.
2016Rick Santorum, Chris Christie and Jeb Bush are the three leading contenders for the Republican nomination. Santorum manages to split the moderates and comes out the winner in Iowa, South Carolina and ultimately winning the nomination. He selects former Virginia Governor Bob McDonnell as his running mate. (And Santorum is next in line from 2012 after all)
On the Democratic side, Hillary Clinton enters the race and sweeps to victory pretty much unchallenged for the nomination. Clinton selects Montana Governor Brian Schweitzer as her running mate.
Nationally, President Obama remains relatively popular but nothing to brag about. The economy is still expanding and getting better but growth remains lackluster at best. People are more confident about the future than they were four years ago. Unemployment is around 6.5%. The US is out of Iraq and Afghanistan. Iraq remains stable, Afghanistan is unraveling quickly but quietly.
Best Case Scenario for Santorum:
Clinton: 371
Santorum: 167
Best case scenario for Clinton:
Clinton: 443
Santorum: 95
Edit: The real best case scenario for Clinton is a 50 state sweep.