TAMPA, Fla. -- Armed with a sheaf of Obama fliers and a computer map of a sprawling, charmless apartment complex near Busch Gardens, Joan Thurmond knocks on one more door.

She's spreading the word that it's Election Day in Florida. So was yesterday. Tomorrow, too. Early voting began last Monday in the Sunshine State and all week voters braved long lines across the state to register their presidential choice.

That's why Sen. Barack Obama opened the week addressing big rallies here with Sen. Hillary Rodham Clinton, and why Sen. John McCain barnstormed the state Wednesday. Jill Biden was in the Tampa area yesterday. Gov Sarah Palin is to address a rally at the city's convention center today.

Ms. Thurmond's job is to make those voting lines still longer. Her mission is not one of persuasion. Her map directs her to the homes of voters who the Obama campaign believes are leaning toward their candidate. But the campaign doesn't want to take any chances on changed minds, rescheduled work shifts, or forgetfulness.

A woman answers her knock at one door of the Garden Terrace Apartments, a little girl peeking out from behind her legs. Mrs. Thurmond verifies the woman's Obama allegiance and explains the early voting procedures.

Walking across the parking lot, a man approaches to show her a barely legible voter registration card. Will he need a new one?

"Don't worry about that, just show your driver's license," she says, making sure he knows the location of the nearest polling site.

Ms. Thurmond is a social studies teacher from across the bay in St. Petersburg. Asked if her name is spelled like the late senator's, she says, "My ex-husband's grandfather was a slave on the Thurmond plantation in South Carolina."

Now, she hopes, she's working to elect the next president. She's one cog in the vast get-out-the-vote machine the Obama forces have put together in this crucial state. They're up against a practiced Republican machine that's kept the statehouse firmly in GOP control and twice helped deliver the state's 27 electoral votes to President Bush, albeit once with a detour through the U.S. Supreme Court.

Fresh from a victory in South Carolina, and buoyed by a timely endorsement from the state's popular governor, Charlie Crist, Mr. McCain gained a virtual lock on the GOP nomination with a victory in the state's primary. Through the summer he held a comfortable lead in the polls over Mr. Obama.

Over the last month, however, Mr. Obama surged ahead. Some recent polls show the Republican clawing his way back, and both sides say the race will be close. And the streets Ms. Thurmond walks are at the center of a region that could tip the balance either way.

Brad Coker, managing director of Mason-Dixon Polling & Research, points out that the counties now rooting for the Rays in the World Series offer a microcosm of Florida's vote as a whole. In recent elections, the vote totals in the counties surrounding Tampa Bay -- Hillsborough, Pinellas, Polk, Pasco and Hernando -- have tracked within a point either way the statewide percentages.

In 2000, Florida's vote split, with both Mr. Bush and former Vice President Al Gore hovering at 49 percent. In the Tampa Bay region, it was Bush, 49 percent; Gore, 48 percent. In 2004, Mr. Bush, boosted by a robust GOP turnout operation, won the state, 52 percent to 47 percent. In the Bay counties, his margin was 53 percent to 46 percent. Recent statewide races for governor and senator have shown the same congruence.

"It has an ethnic and racial mix that mirrors Florida and the nation at large," said Dr. Susan McManus of the University of South Florida, describing this region and the adjacent I-4 corridor extending to Orlando.

"There's a pretty even split between young, middle-aged and older voters. You have all the geographies of the state and the nation reflected here -- rural, urban and suburban, everything from citrus growers to Tampa. ... It's the most important piece of political real estate in Florida -- the bellwether of the bellwether state."

The Tampa region mirrors the nation's diversity to the extent that it's frequently used as a test market for national products and firms ranging from Neiman-Marcus to Circuit City to restaurant chains.

"You've got old Floridians, a lot of transplants who came down I-75 from the Midwest," Mr. Coker said. "Hillsborough [County] has a mix of black and white; you've got a nice mix of Hispanics -- the old pre-Castro Cubans in Ybor City, and a lot of newer immigrants."

Ms. McManus notes that age represents one important change in the churning political demographics of this dynamic state.

"Right now, the numbers of voters under 35 exceed those 65 and older," she said. "That's a pretty drastic shift, and a lot of that is due to the aggressive registration of young voters by Obama."

As in Pennsylvania, that registration effort has more than doubled the Democratic advantage in the state. The party's edge on the voter rolls statewide is now more than 660,000, compared to 312,000 a year ago. Obama officials say they have had conversations with 1.3 million voters since Labor Day/

While most of the Obama volunteers are local, the scale of the campaign's effort is reflected in the geographic range of some of its other foot soldiers.

Mridu Sekhar, who lives in Mr. Obama's Hyde Park neighborhood in Chicago, and recently sold her export business, came to Florida in late September.

"I told them I wanted to be where it counted," she said as she stood in a light intermittent rain outside a polling place. Now, she helps man Casa Obama, a campaign headquarters in a Hispanic neighborhood in West Tampa, and is occasionally dispatched to outreach efforts for Asian-Americans and Pacific Islanders.

Across town, Ian Van der Kooye, a Dutch citizen, hands out Obama literature outside another library enlisted as a polling place. He came here on vacation, because, "This is history."

Mr. Van der Kooye said he E-mailed and phoned the Obama campaign from his Amsterdam home and was told, "We could use all the help we can get in Hillsborough County."

Jim Greer, the chairman of the state Republican Party, acknowledges the reach of the Obama effort, but says he's confident that the experienced GOP cadres will more than match it in effectiveness.

"The Republican Party and the McCain campaign in Florida have an excellent ground game all across the state," he said. "I think there's some percentage of this Obama ground game that's a little bit of a phantom. They're all over TV, but at the end of the day, the Republican Party has always had a successful [get out the vote] effort and I believe that is going to prevail."

An analysis compiled by the Democratic campaign from the running tallies kept by county elections officials showed that after the first four days of early voting here, Republicans had a narrow edge in total votes cast -- early and absentee -- of 651,700 to 650,363, with another 225,980 votes from independent or third party voters.

Jay Carson, an Obama aide, predicted on Friday's conference call, however, that the Democratic numbers would overtake the Republican tally by tomorrow. But so far, in a reflection of the traditional organization of the GOP here, the ratio of Republican early votes to Democratic ones is closer to parity in Florida than in other contested states focused on by the Democrats.

The Obama camp said that in his campaigning here last week, the Democratic nominee attracted crowds totaling more than 80,000 in rallies in across the state.

Mr. McCain's crowds, while enthusiastic, were significantly smaller. Roughly 3,000 partisans cheered him in Sarasota Wednesday, although Eric Robinson, the county GOP chairman pointed out that a long line of supporters had to be turned way because the local party hadn't been able to find a larger venue in the time they had to prepare for the "Joe the Plumber" tour.

"Then, I'm the bad guy," he said, as he eyed a fire marshal poised to cut off entry to the SRO event.

Mr. Robinson maintained that the GOP organization would deliver the state to Mr. McCain despite the Democrat's seeming advantage in money and manpower.

"People are very, very energized," he claimed. "[Vice presidential nominee Sarah] Palin was a real shot in the arm, and now people are responding to the Joe the plumber stuff. Crist has been very active; we had him here Monday.

"You read the paper; you watch TV and it brings you down, then you go in the office and you see the energy -- there's a disconnect."

A Republican partisan watching Florida television might well be discouraged. Statistics released by the Nielson Co. last week showed the Democratic campaign vastly outspending the Republicans in recent weeks.

In an analysis of swing state advertising, Nielson stated "Obama's advertising has been most prolific in Florida, where he ran 15,887 ads between Oct. 6 and Oct. 22, 2008, outpacing McCain's 4,662 ads by 240%."

David Plouffe, Mr. Obama's campaign manager, contended that those numbers didn't tell the whole story as they omit spending by the Republican National Committee as well as hybrid ads whose cost is shared by the RNC and the McCain campaign. Nonetheless, a few hours of attention to local stations here shows a clear preponderance of Democratic ads.

While the numbers weren't as big, the Obama campaign was outspending the Republicans on Florida television through the summer as well, without making an appreciable dent in the McCain lead.

That post-convention edge led senior McCain officials to crow that the Obama camp had made a strategic blunder in concentrating so many resources on the state. But as Mr. Obama's poll number surged nationally in late September, the McCain advantage steadily eroded here. By the first week of October, the polling consensus had swung to the Democrat.

A St. Petersburg Times/Miami Herald survey this week showed Mr. Obama with an increased margin of 49 percent to 42 percent. But other recent surveys show the pendulum beginning to swing back toward the Republican.

In contrast to the Times/Herald results, the latest Mason-Dixon survey showed Mr. McCain essentially tied statewide, with 46 percent for Mr. McCain and 45 percent for Mr. Obama. Following Florida's traditional voting patterns, the state's north -- culturally, its most Southern region -- produced a big margin for the Republican while the populous southeast was Obama country. In the Tampa Bay, the battleground state's battleground, Mr. McCain led 47 percent to 44 percent.

At a time when national polls have tilted strongly in Mr. Obama's direction, a poll of bellwether Hillsborough County, conducted by Insider Advantage for Politico, showed similar recent movement toward Mr. McCain.

State and national Republicans concede that without Florida, Mr. McCain has no plausible path to an electoral vote majority. The Bay area's residents have helped determine the fates of quite a few products over the years. This time they could be the decisive test market for the next president.