CLAIRTON, Pa. (AP) -- Five days a week, Linda Graham trolls tattered neighborhoods of this once thriving steel city outside Pittsburgh for unregistered voters she can sign up as Democrats - one of thousands of unknown volunteers whose work outside the limelight has already altered the basic arithmetic of the November election.

The epic nomination battle between Barack Obama and Hillary Rodham Clinton helped put millions more Democrats on the voter rolls while Republican registration declined. Now Graham, 45, has taken three months of unpaid leave from her job at Pittsburgh's Central Blood Bank in the hope of adding to those gains before the presidential vote.

She's encouraged by the response here. "They're all feeling the crunch" of lost jobs and a sagging economy, Graham said. "But people are feeling empowered. They're feeling like, you know what, I hold a little bit of power in this."

To counter this effort, the Republicans are counting on a formidable, high-tech get-out-the-vote operation that has helped them win the past two presidential elections.

Since the last federal election in 2006, volunteers like Graham combined with the enthusiasm generated by the Obama-Clinton struggle to add more than 2 million Democrats to voter rolls in the 28 states that register voters according to party affiliation. The Republicans have lost nearly 344,000 thousand voters in the same states.

The Democrats hope their voter registration efforts can boost Obama to victory in competitive states like Pennsylvania, Nevada and Florida and perhaps even give him a shot at winning traditional Republican states like Georgia, North Carolina and Virginia.

Both Obama and his Republican rival, John McCain, are fighting for independent swing voters, and many of the new Democrats had been unaffiliated voters.

The number of unaffiliated voters dropped by nearly 900,000 since 2006. Many joined the Democratic Party to take part in the primaries and caucuses, and now they will now be targeted by an aggressive get-out-the-vote campaign.

"We feel that our supporters are more enthusiastic than we've seen in previous cycles," said Jon Carson, Obama's national field director.

The Obama campaign is taking the lead among the party organizations and labor unions that traditionally work on voter registration efforts.

Because party organizations and unions, like the Service Employees International Union to which Graham belongs, can raise unrestricted amounts of money, presidential campaigns typically rely on them to handle the bulk of voter registration drives, Democratic National Committee chairman Howard Dean said in an interview.

"This is the first campaign I've seen where the voter registration is done by the campaign," Dean said.

The Republicans are relying on a more traditional voter registration model, with the Republican National Committee leading the effort among state parties.

"We hope that the hard work we've done in the past will provide us with a strategic advantage," said Mike DuHaime, McCain's political director. "We will have the most technologically advanced ground operation ever."

DuHaime said the RNC is working with the state parties to register voters in every battleground state. He said there is extra emphasis on the fast-growing ones, including Nevada, Colorado, New Mexico, Florida and North Carolina.

He said the GOP's comprehensive voter database helps it track voters moving into competitive states.

"If you ever voted in a Republican primary and move without registering, we pick it up," DuHaime said.

Nationwide, there are about 42 million registered Democrats and about 31 million Republicans, according to statistics compiled by The Associated Press.

The Democrats have posted big gains in many competitive states, including Nevada, New Hampshire, Iowa, Colorado and Florida. They have also been targeting historically Republican southern states.

Since 2006, the Democrats have added 167,000 voters in North Carolina, while the Republicans have added 36,000. The Democrats' biggest voter registration goal is in Georgia, where the Obama campaign hopes to register 500,000 voters before the election, said Dean, who has spent the past month traveling the country on a voter registration bus tour.

"The Obama folks are serious about Georgia," Dean said. Georgia has added 337,000 voters since 2006, but the state does not identify them by party affiliation.

In Pennsylvania, the Democrats have added 375,000 voters since 2006 while the Republicans have lost 117,000.

America Votes, an umbrella organization, coordinates voter registration efforts for more than 40 groups in Pennsylvania, including unions, the NAACP and the Sierra Club.

On a recent weekday, two dozen volunteers canvas neighborhoods in five southwest Pennsylvania counties, targeting African-Americans in their teens and twenties, who tend to vote at lower rates than older voters.

Graham, the SEIU member, works the neighborhoods of Clairton, where the steel industry's decline has left more downtown storefronts boarded up than occupied.

Graham finds a potential voter at the first house she stops at. Justin Webb, a father of two, is unregistered, but tells Graham he has serious concerns about the economy.

"We need more jobs," said Webb, 28. "If we had more jobs, we would have more opportunities to better ourselves."

It takes Graham less than five minutes to register Webb as a Democrat.