RCP has included another national poll that asks their sample who they voted for in 2008, from the Hartford Currant/University of Connecticut. They give Obama a 3 point edge, 48-45.
I've maintained that the Republicans and Romney are in a much improved position over 2008. This poll actually gives support to this contention, showing that McCain voters are going for Romney 95-5 compared to 2008 Obama voters, who are only going to Obama by 86-10 margin.
In fact, if you compared these percentages to the actual 2008 votes, you would get:
Romney 10% of 69,496,897 plus 95% of 59,934,814 equals 63,887,763 voters.(.1*69496897+.95*59934814=63,887,763)
Obama 86% of 69,496,897 plus 5% of 59,934,814 equals 62,764,072 voters.
So if this poll is correct, and the 2012 election consisted only of 2008 voters, Romney would win by over a million votes!
So how can this poll say that Obama is leading by 3 points? By oversampling Obama voters in comparison to McCain voters. Their sample includes 856 people who say they voted in 2008. Of those, 469 voted for Obama vs 370 for McCain. This indicates an 11.6 Obama margin ((469-370)/856). Obama, of course, only won by 7.2%.
Given that it is instead highly probable that a smaller percentage of Obama voters will vote this year, if this poll had a realistic sample it would show Romney in the lead, probably by a 2-4 point margin.