TALLAHASSEE, Fla. -- The state that helped propel John McCain to the Republican presidential nomination could now be the one that costs him the White House.

The lead McCain had here for much of the year has slipped away largely because of Democrat Barack Obama's aggressive Florida campaign and the public's reaction to bad financial news. That leaves Florida, the largest of the battleground states, a tossup.

Sound familiar?

"Can this thing come down to a thousand votes? Or 537 votes again?" said Brett Doster, President Bush's Florida political director during the 2000 election that took five weeks to sort out. "Hell yeah it can."

McCain looked strong in Florida earlier this year. He has personal ties here from his days in the Navy, is close to the popular governor and is well-liked in the influential Cuban-American community. On top of that, he had a head start campaigning in Florida while Democrats carried their nomination battle into June.

But Obama has poured millions into Florida, far outspending McCain on advertising and building a team that overshadows John Kerry's efforts four years ago and Al Gore's in 2000. Now there are several factors that will shape the race here. Can McCain win over Jewish voters? How will non-Cuban Hispanics vote? Will Obama's efforts to increase black registration and turnout work.

There's also the question of whether the Republicans' strong grass-roots and voter turnout operation President Bush took advantage of four years ago will be as solid this year. And, on the other side, if the grass-roots operation that helped Obama carry states like Iowa on the road to the nomination will work in Florida.

"If his turnout operation works as well as it did in Iowa, Obama is going to win the state," said Matthew Corrigan, University of North Florida political science professor. "Obviously Iowa is a much smaller state."

Florida had more than 10.7 million voters as of August -- almost 4.5 million Democrats, nearly 4 million Republicans and more than 2.3 million voters who aren't registered with either party.

The state is far from homogeneous. Urban South Florida favors Democrats, largely rural north Florida identifies more with the South and votes conservative, and the Interstate 4 corridor that splits the state from Tampa through Orlando to Daytona Beach is where both parties fight aggressively for independents and crossover vote.

Florida has a high population of transplants from the Northeast and Midwest; immigrants from Latin America, Haiti and elsewhere and a large number of Jewish voters. Retirees flock here and there are a lot of veterans, many of whom stay after serving in the state. It means Florida is a difficult place to campaign.

Corrigan also points out that polling is difficult in Florida. Need a reminder? Many news organizations called Florida for Gore in 2000 based on exit polls, only to take it back when it became clear the race was too close to call.

Doster, who also ran Bush's Florida re-election campaign in 2004, when the president won by a more comfortable 381,000 votes, said this year's election should look a lot more like 2000 than 2004.

"Even though Republicans are in the White House and a Republican is in the governor's mansion, this is definitely different than it was four years ago," said Doster.

In 2004, Florida's economy was still soaring. Unemployment was low, the housing market hot. Jeb Bush, the president's brother, was a popular sitting governor and Republicans were winning the argument with Florida voters on national security and Iraq.

Now the economy is the top topic in the election and Florida's unemployment is higher than the national average and the highest it's been in more than 13 years. It has one of the worst foreclosure rates in the nation and the seniors who may have favored McCain may not like the idea of another Republican in the White House as their retirement accounts get battered by the financial mess on Wall Street.

Florida was the key to McCain's nomination. He won the state after a last-minute endorsement from Gov. Charlie Crist and used the momentum going into Super Tuesday a week later. McCain quickly came to Florida after locking up the Republican nomination, visiting the Pensacola and Jacksonville areas - places he had served in the military and regions that firmly support Republicans.

Florida's 27 electoral votes, 10 percent of the total needed to win, is also McCain's key to winning the White House.

"Without Florida, McCain is not gong to make 270," said Corrigan. "Florida is not make or break for Obama."

Obama has come a long way in Florida in a short time. He refused to come to Florida for the four months leading up to the Jan. 29 primary because the early date violated party rules. He then focused elsewhere as he and Hillary Rodham Clinton fought for the nomination until June.

But his campaign began airing television ads in Florida weeks ahead of McCain. It also quickly built up a large operation, going to parts of north Florida like Panama City that Democrats usually avoid and opening offices in poor, rural communities like Pahokee and Quincy that usually don't get notice from any candidates.

The difference in how Florida is responding to Obama compared to Kerry's 2004 run is significant, said Kirk Wagar, Obama's Florida financial chair and a top Democratic fundraiser in 2004.

"This doesn't come as a surprise to me that we've got the energy in Pahokee and Panama City," Wagar said. "It's just because the energy we've had on the fundraising and the grass roots and the house parties and things that we've done has been so deep and so wide."

But McCain's lead didn't disappear until the nation's financial meltdown, a fact that the McCain campaign points to as an argument that Obama's ad blitz and large staff weren't changing numbers, but rather it was the downturn.

With about three weeks to go in the election, Obama had spent more than $21 million in Florida on the general election, compared to about $5 million spent by McCain, according to TNS Media Intelligence/Campaign Media Analysis Group, a firm that tracks political advertising.

"I would say that we're feeling urgent," said Obama deputy campaign manager Steve Hildebrand, who is now working out of Miami. "We've got a lot to prove. We built a big organization, but now we've got to make sure the vote turns out. We feel good that is going to, but it's not a given."

Republican Party of Florida Chairman Jim Greer acknowledges that the McCain campaign may have overestimated their strength here.

"There was some assumption that Florida was going to be fairly solid on their side and maybe they didn't anticipate this being as close," said Greer, who began a tour of campaign offices around the state to motivate workers, a move made after some internal tension as the polls dropped.

But Republicans clearly aren't conceding the state, which has only supported one Democratic presidential candidate since backing Jimmy Carter in 1976 - Bill Clinton in 1996. Even then, Clinton only received 48 percent of the vote.

"We have good people, both staff and volunteers, who have been here before. They've been in tough elections, close elections every two years for 10 years," said Mike DuHaime, McCain's political director. "(Obama's) got more money, he's got more staff, he's got more offices. We've got a better candidate. In terms of the turnout operation, we've got better technology and a battle-tested organization."

He also pointed out that despite Democrats advantage in voter registration, Republicans have requested about 200,000 more absentee ballots.

"Right now the thing that I feel the best about Florida, we have a pretty sizable absentee ballot lead," DuHaime said. "When you start looking at the data of real people actually voting, I feel good about it."