History: Third Party More Likely Than Romney

Mitt Romney will probably not win either of his 'home' states in the 2012 General Presidential election.  Romney counts Michigan as his birth home, and Massachusetts as his resident home state.  He is impossibly behind his Democratic opponent in both of them.  Historically, this makes it extremely unlikely that he will win the overall Electoral College vote.

Only one President has ever won the race without winning either his home or adopted home state.  It happened almost 170 years ago.  James Polk won the 1844 Presidential Race without winning either state.  The Liberty Party candidate shaved off just enough percentage points from the Whig Party candidate to give Polk the win, even though he couldn't manage to win his home or resident state. 
Romney doesn't really have this going for him.

Romney will need to win not only all of the southern states and mid-west, but he'll need to win the swing states of Florida and Ohio.  Currently, he is almost 10 points behind in these key swing states.  More surprisingly, he barely leads in some of the traditional GOP stronghold states.  Winning the populous Michigan electors would make up for some of this, but that doesn't look plausible.

Those who are against going to a third party candidate because "they want to back a winner" would have to back Obama in this race, unless they believe they can somehow overcome presidential election history.  A third party candidate has been elected more recently than a candidate who couldn't win his home states.  His name was Abraham Lincoln and he was elected as the 3rd party candidate to a fledgling political organization called the Republican Party sixteen years after Polk was elected.