Here is the actual number in case anyone cares
131 million voters cast their ballots in the 2008 election in which Barack Obama defeated John McCain by a 53% to 46% margin. Obama received 69.4 million votes, while McCain received 59.9 million.
In 2012, Obama defeated Romney by a 50% to 48% margin. Obama received 59.8 million votes, and Romney received 57.1 million votes -- 2.7 million fewer than Obama in 2012, but also 2.8 million fewer than McCain in 2008.
So 3 million registered republicans and those who voted third party who did not vote could have made a big difference.