Obama beat McCain by 53-46, seven points. That was a little over 9 million votes. So Romney needs to either get 4% of Obama's voters to switch; or he needs to get some to switch and get more Republican turnout and less Democratic turnout. He needs five million more, and he needs Obama to get five million less. It's that simple, and it's very doable:
1. In 2008 a crashing economy was blamed on the then-current President, a Republican. In 2012 there's no Bush on the ballot. Whatever is happening now in the economy, it will be blamed on Obama.
2. In 2008 the Democratic electorate was energized by the opportunity to vote for the first-ever African-American Presidential candidate. They won't be as energized in 2012, not even close.
3. In 2008 the Republican candidate was a tired re-tread, McCain. Romney is better already, and he's getting better too.
4. In 2008 Obama was hailed as a charismatic speaker, an orator of Lincoln/Churchill/MLK stature. Does anyone think that anymore? Aren't a lot of people embarrassed by the gushing over this lightweight? Even if that's too harsh, will his act really play as well the second time around?
5. In 2008 Obama ran as someone who would unite America and pacify the world through the strength of his personal magnetism. After four years of divisive, mean-spirited, us-against-them rhetoric, will anyone argue that Obama is a bipartisan uniter? With Russia sending ships to Syria and Israel poised to attack Iran's nuclear labs, does anyone think the world is safer? That America's reputation abroad is stronger?
6. Obama's race, charisma, oratory skill, and the adulation of the media made him hard to dislike in 2008, or at least hard to dislike openly. Republicans were understandably disheartened, and didn't turnout to vote in the same numbers they did in 2004 or 2000. Obama's easy to dislike now. Republicans despise him and everything he stands for -- elitist academic know-nothing leftism. They will turn out in droves to throw him out.
So put aside the issues -- Fast and Furious, Solyndra, bowing to foreign leaders, $5 trillion in new debt, Obamacare, tax demogoguery, 100 rounds of golf while there's 8% unemployment -- put all that aside. Just structurally:
Obama isn't the Messiah anymore. We know him. We've taken his measure.
Romney isn't McCain.
2012 isn't 2008.