2010 may be a good year for Republicans on par with the great year Democrats had in 2006. But the reason for their likely victory is very different.
The Democrats' big win in 2006 was not driven by the enthusiasm gap, but because a lot of people who had voted for George W. Bush in 2004 switched over to supporting Democratic candidates. According to the 2006 exit poll the electorate that year was actually more heavy on Bush voters than the 2004 electorate that reelected Bush. 49% were Bush voters to only 43% who were Kerry voters, compared to Bush's 51-48 popular vote victory in 2004.
The reason Democrats won even though the electorate disproportionately consisted of Bush voters was that 15% of those Bush voters cast their ballots for a Democrat, a pretty large amount of crossover.
There aren't nearly that many Obama voters leaning toward the Republicans this year. Our last national generic ballot survey found only 8% of people who voted for the President in 2008 were planning to support the GOP this year. But those surveyed represented an electorate that favored Barack Obama by only a point, 46-45, and because of that the generic ballot was tied despite the small number of voters crossing over.
The big 2006 Democratic win was about voters abandoning the GOP. If Republicans have a big win in 2010 it's mostly going to be about Democrats staying home. And it's important to keep that in mind when considering the implications of the 2010 results for 2012- Barack Obama might be alright in 2012 even if his party gets pummeled this year if he get those folks back out to vote when it's him on the ballot.