WASHINGTON - In 2008, Barack Obama tapped into a record of nearly 15 million voters who cast ballots for the first time, a surge in registration that may be difficult to replicate next year.

Recent voter-registration data show Democrats have lost ground in key states that Obama won in 2008 - including Pennsylvania - an early-warning siren for the president's reelection campaign.

While Republican numbers have also dipped in some states, the drop in the Democrats' ranks highlights the importance of the Obama campaign's volunteer base and the challenge they could have of registering new voters.

Democratic strategist Chris Lehane said the political map in 2012 is likely to look more the way it did going into the close contests of 2000 and 2004, which hinged on swing states such as Florida and Ohio, than in 2008, when Obama won traditionally Republican states such as Indiana and North Carolina.

Obama will have to reignite the passions of some Democrats who may be ambivalent about him now. Several states with Republican governors have tried to reduce the number of early voting days and required photo IDs, a move that Democrats say will disenfranchise poor and minority voters.

Polls have also shown some political independents drifting away from Obama since 2008, meaning Democrats need to register and turn out more Hispanic and black voters, college students, and women.

While Democratic registrations ballooned before the 2008 election, the numbers have declined in several important states, including:

Pennsylvania. Democrats maintain a 1.5 million voter advantage over Republicans in registrations, but their numbers have dwindled since Obama's election. There were 4.15 million registered Democrats through mid-May, compared with 4.48 million in 2008. Democrats added about a half-million voters to their rolls in the two years before the 2008 election. Republicans now have more than three million registered voters, compared with 3.2 million in 2008. About 500,000 Pennsylvania voters are unaffiliated.

Florida. Democrats added more than 600,000 registered voters from 2006 to 2008, giving Obama about 4.8 million registered Democrats to help his cause. Registered Democrats now number 4.6 million in the Sunshine State. Republican registrations slipped from 4.1 million in 2008 to 4.05 million in mid-March, the most recent data available. Nearly 2.6 million Florida voters are unaffiliated.

Iowa. Republicans have gained ground in the state that launched Obama's presidential bid. GOP registrations increased from about 625,000 voters in 2008 to nearly 640,000 in early May. Democrats fell from about 736,000 voters in 2008 to 687,000 in May. Nonpartisan voters remain the largest bloc in the Hawkeye State, representing more than 762,000 voters.

Democrats' numbers have also fallen in North Carolina, which Obama was the first Democratic nominee to carry since 1976, and Nevada, a high-growth state battered by the recession.

Several Democratic-friendly cities have not been immune, either. Philadelphia had 880,000 registered Democrats in 2008; that number has fallen below 800,000. Denver, where Democrats held their 2008 convention, had about 200,000 registered Democrats in November 2008, but that's now down to about 120,000.

Obama officials said voter registration would be a top priority. Adviser David Axelrod said the campaign would "mount a major effort, and it's not just about registering new voters but it's also reregistering people who have moved because there is a high degree of transiency among young people and often among minority voters."

Ben LaBolt, an Obama campaign spokesman, said the president "has demonstrated a consistent ability to reach new voters and voters who don't identify as Democrats, so party affiliation isn't the only factor to evaluate."