Most political observers are generally aware that Democrats have been benefitting from a surge in party registration this year. But Rhodes Cook has offered a clear statistical look at the Democratic registration advantage, going back to the 2004 election.

Keep in mind that only 29 states (plus DC) register voters by party. So Cook's national numbers--a total increase in Dem registration of about 700,000, and a decline in GOP registration by about a million--just show part of the picture.

But far more significant are the trends in some of the battleground states. Combining D and R numbers, the net shift towards Democratic as opposed to Republican registrations since November of 2004 has been 124,000 in Oregon, 94,000 in Iowa, 60,000 in both Colorado and Nevada, 33,000 in New Hampshire, and 30,000 in Arizona. But it's the trend in Pennsylvania--a battleground in both the presidential and House races--that jumps off the page: Democratic registration is up 266,000 since '04, while Republican registration is down 220,000. That's a net shift of 486,000; Democrats now enjoy a plurality in registrations of more than a million in PA.

A number of big battleground states (notably OH, MI, VA, MN and WI) don't register voters by party. But you'd have to guess the underlying partisan dynamics don't differ massively from those in the party registration states. And that's a major reason for Democratic optimism this year.