WASHINGTON (AP) - 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 that Democrats have lost ground in key states that Obama carried in 2008, an early warning siren for the president's re-election 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.

"When you look back at 2008 there has to be a recognition that it was a historic election, a historic candidate, a historic moment in time and potentially some type of a ceiling - I'm not sure there is ever a hard ceiling - in terms of voter registration," said Democratic strategist Chris Lehane. He said the political map in 2012 will likely look more like it did going into the close contests of 2000 and 2004, which hinged on swing states like Florida and Ohio, respectively, than in 2008, when Obama won traditionally Republican states like Indiana and North Carolina.

Obama will have to re-ignite the passions of some Democrats who had high hopes going into his presidency and 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 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 prior to the 2008 election, the numbers have declined in several important states, including:

- Florida: Democrats added more than 600,000 registered voters between 2006 and 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 have slipped from 4.1 million in 2008 to about 4.05 million in mid-March, the most recent data available. Nearly 2.6 million voters in Florida are unaffiliated.

- Pennsylvania: Democrats maintain a 1.5 million voter advantage in registrations over Republicans, but their numbers have dwindled since Obama's election. There were 4.15 million registered Democrats through mid-May, compared with about 4.48 million in 2008. Democrats added about a half-million voters to their rolls in the two years prior to the 2008 election. Republicans currently have more than 3 million registered voters, compared with 3.2 million in 2008. About 500,000 Pennsylvania 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, meanwhile, have fallen from about 736,000 voters in 2008 to about 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, where Obama became the first Democratic nominee to carry the state since 1976, and Nevada, a high-growth state that has been 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 - that's now down to about 120,000. In Mecklenburg County, N.C., whose county seat, Charlotte, is the site of the 2012 Democratic National Convention, Democrats' numbers have fallen after major gains leading up to the 2008 election.

Obama officials said voter registration will be a top priority. Obama 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. We want to make sure that not only new voters but people who have moved are registered again."

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. The campaign's efforts to expand the electorate to new voters and voters with less consistent voting histories was one reason why the president was elected in 2008, and as we continue our organizing efforts it's certainly something we'll take into consideration."

Finding new voters has been a longstanding goal of Obama, who ran a successful voter registration drive in Chicago when Bill Clinton sought the White House in 1992. Sixteen years later, Obama's campaign was fueled by a massive grassroots campaign and advocacy groups who registered millions of new voters and then turned them out in record numbers.

In a strategy video released in April, Obama campaign manager Jim Messina noted that Democrats registered about two-thirds of the new voters in 2007 and 2008 in states that allow for party registration. Obama, in turn, won nearly 70 percent of the nearly 15 million first-time voters in 2008.

"That made real differences in very close states across this country. We've got to do that again in 2012," Messina said.

Both political parties maintain private voter databases that allow them to closely monitor registration changes, but public data is more difficult to ascertain. Nationally, more than half the states allow registered voters to indicate a party preference when registering while many states in the South and Midwest don't provide for a party preference.

Voter registration and turnout were critical to the last president running for re-election - George W. Bush in 2004. Bush's operation registered an estimated 3 million new voters, helping it drive up vote totals in the areas straddling key suburban regions in Florida and Ohio.

Blaise Hazelwood, who ran the Republican National Committee's voter registration effort in 2004, said campaign officials pored over Excel charts tracking new registrations on a daily basis and used the mail, door knocking and supermarket stands to find voters in places more inclined to support Bush. She said it would be difficult for Obama's operation to replicate 2008.

"There's no way they can get all those voters back," Hazelwood said.

(Copyright 2011 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)