Sean Trende, writing at RealClearPolitics: 

[One] can readily see that 2012 is simply not a year like 2008 or 1980, which stand as the worst or second-worst presidential years in recent history in almost every category. Nor, of course, is it a year like 1984, 1964, or 1972, when the economy was going full-bore ahead. We also see 1968 stand out as a year where the economy was strong, but outside forces disrupted what should have been an easy re-election effort for the party in power. Instead, this is a middling sort of year, like 1960, 1976, 2004, or 1992, where the economy is limping along, but not contracting. Those years have generally produced close elections. In fact, those elections describe what I see as something of a “95 percent confidence interval” for this election – i.e., I’m 95 percent sure the result will fall in between these elections -- a 4-to-5-point Romney win (a la 1992) or a 2-to-3-point Obama win (a la 2004).  So think of it this way.  In 1980, Jimmy Carter didn't have an argument for re-election that appealed very far beyond the Democratic base.  Similarly, in 1984, Walter Mondale simply didn't have much of an argument for getting rid of Ronald Reagan.  The Republicans didn't have a good argument for holding on to power in 2006, nor did the Democrats in 2010.  The elections reflect that. This year, Barack Obama has an argument -- he didn't inherit the mess, and the economy is slowly expanding.  That's an argument that is probably good enough to get him to 46 or 47 percent of the vote.  Similarly, Mitt Romney has a pretty good argument for electing a new president, one that will shore up his base and Republican-leaning independents.  Thus, we should probably expect what we're presently seeing in the polls: a close race, to be decided by a relatively small slice of the electorate.

Whole thing here.