A theory for your consideration:
There have been more than 50% of the electorate that would like to fire Obama. All they needed was a plausible alternative...someone they believe could be President. Along comes Romney in the first debate and hits the ball out of the park. The comparison of the two candidates was stark. Millions of voters who thought Romney was what the Obama campaign machine had portrayed him to be...rich, out of touch and clueless. Imagine their surprise when they met the real Romney!
So, Romney passes the "plausible alternative" test with flying colors and the polls prove it. Debate #2 only confirmed the impressions from Debate #1. (btw...Ryan did the same) Those 50%+ will provide a near landslide.