The enthusiasm gap is the dominant feature of the 2010 electoral landscape. The split has two basic components. First, you have some discontent among liberal voters who thought that President Obama's election meant that his entire agenda would sail through uncompromised, or possibly even enhanced, by the legislative process. It is hard to gauge exactly how important this dynamic is. Conservative activists tend to worship Republican presidents when they hold office, and then only after they have definitively failed, they throw them overboard and declare them heretics to the cause. Liberal activists go through the process quicker, moving through the worship phase a few days after inauguration and into the burn-the-heretic phase by about three months into the presidency.

How sizable is liberal discontent, asks Chris Bowers? Pretty sizable:

From 2008 to 2010, President Obama has suffered far more erosion of support among self-identified liberals than among self-identified moderates or conservatives:
  • In 2008, according to exit polls, 89% self-identified liberals voted for President Obama.  Over the past four weeks, according to Gallup, President Obama's approval rating among self-identified liberals has averaged 74%. That is a decline of 15 points.

  • In 2008, according to exit polls, 60% of self-identified moderates voted for President Obama.  Over the past four weeks, according to Gallup, President Obama's approval rating among self-identified moderates has averaged 54%.  That is a decline of 6 points.

  • In 2008, according to exit polls, 20% of self-identified conservatives voted for President Obama. Over the past four weeks, according to Gallup, President Obama's approval rating has averaged 24% among self-identified conservatives.  That is an increase of 4 points.

So, according to Gallup, disapproval among self-identified liberals accounts for the majority of President Obama's approval rating underperformance compared to his 2008 vote share (from the perspective that the smaller decline among moderates is partially canceled out by the small gain among conservatives).  If it were not for President Obama's decline among liberals, there would be virtually no difference between his 2010 approval rating and 2008 voter performance.

I'm a little skeptical of these numbers because ideological self-identification is a poor tool for gauging voter ideology. The labels don't match up well with the actual issue preferences of voters. Still, this does show that at least something is happening here in terms of liberal discontent.

The second aspect is the old-ification of the Republican Party. 2009-2010 have been dominated by a health care debate that splits oldsters, who despise health care reform, from all other age groups, who approve of it. The generational split seems to have deepening partisan overtones. In the long run, this is a boon for Democrats. In the short run, it's a disaster, because old people vote in midterm elections and young people don't:

Exit poll data shows that the electorate skews older, particularly in nonpresidential election years. In 2006, 63 percent of those who cast ballots were 45 or older, and in 2008, that same age group made up 53 percent of the electorate, according to exit poll data from the National Election Pool. This group made up about half of the adult population in those years.

The thread connecting both these factors is a belief that the presidency is the end-all, be-all of American politics. Young people and liberals get very excited about presidential elections. When the other party holds the White House, they can get fired up about turning them out. When their party wins, they expect all their problems to be solved. When the problems don't disappear immediately, they get disillusioned.

To some extent, this dynamic can be found on both sides. But it seems especially sharp on the left. It happened in 1994, and it's likely to happen in 2010. It's a deadly combination for Democrats.