The last unknown in White House race -- who votes?

WASHINGTON (Reuters) - It is the last great unknown in a White House battle that has been polled and analyzed for nearly two years -- who will actually show up and vote?

The outcome of the race between Republican John McCain and Democrat Barack Obama rests on which unpredictable combination of new voters, young voters, black voters, suburban voters, white voters, Hispanic voters, rural voters and even sporadic voters cast a ballot in Tuesday's election.

Officials in both campaigns are predicting a record high turnout -- and a surge of new voter registrations and the long lines for early voting across the country appear to confirm those predictions.

But uncertainty about the makeup of the new electorate, and whether that high turnout could shatter historic voting patterns or simply reinforce them, casts doubt over opinion polls showing Obama with a solid lead on McCain.

"The question of who turns out to vote is the key to who becomes the next president," said Steven Schier, a political analyst at Carleton College in Minnesota.

Obama is hoping a big boost among new and sporadic voters, particularly blacks and the young, will propel him beyond the total of 56 million ballots for Democrat John Kerry in 2004.

But McCain campaign officials said there is no evidence a higher turnout will fundamentally change the makeup of the electorate -- or, they hope, the outcome on Tuesday.

"There is no question that turnout is going to be high," McCain pollster Bill McInturff said, predicting 130 million to 135 million voters by Tuesday -- about 10 million more than 2004 and 25 million more than 2000.

But he said turnout appeared to be higher in all demographic groups, potentially wiping out any advantages Obama gains from an increase in black and young voters.

"They make this race difficult to predict," McInturff said of the new voters, "but overall very, very close."

Obama officials are encouraged by the results of early voting. Officials in battleground states like Florida, Colorado, New Mexico and North Carolina report more Democrats than Republicans have cast early ballots, in some cases by bigger margins than in 2004.

In Colorado and North Carolina, the number of early votes already cast is more than half the total number of votes in 2004. In the vital showdown state of Florida, early voting hours have been extended and more than one-third of the 2004 total already has been cast.

'THE DIE IS CAST'

"The die is being cast as we speak. We think we've built up advantages in all of those states," Obama campaign manager David Plouffe said. "So Senator McCain on Election Day is going to have to not just carry the day but carry it convincingly."

In Nevada, he said, 43 percent of Democrats who voted early are new or sporadic voters, and 19 percent of the Democrats who voted early in North Carolina had never voted in a presidential election before.

"We're kind of out of the land of theory in a lot of these states. We're beginning to see how the election is likely to unfold based on the early voting pattern," Plouffe said.

In Florida, which decided the 2000 election, Plouffe said Obama was making headway.

"We are doing much better with Hispanic voters in Florida than, certainly, was the case in 2004 and, we believe, in 2000 as well," Plouffe said.

"We are doing very well with Puerto Rican voters, Colombian voters. We're doing, I think, surprisingly well with younger Cuban voters," he said. Cuban voters are traditionally more conservative and tend to vote Republican.

But McCain campaign officials said Republicans, who usually lead among absentee voters, typically trail in early voting figures.

"We are used to operating in an environment where we have to pick up Democratic and independent support," McCain's political director Mike DuHaime said. "We are actively turning out conservative Democrats and independents who we believe will vote for John McCain."

While there is evidence black turnout has increased, young voters are less predictable. An Orlando Sentinel analysis of early voting in Florida found blacks made up 22 percent of those casting ballots before election day, even though they are just 13 percent of the state electorate.

But young people turned out in disproportionately low numbers, with only 15 percent of early voters under the age of 35. That group makes up 25 percent of the electorate.

"I think Obama needs both groups and he needs them in big numbers, and the youth vote is a real question," Schier said.

Both campaigns will launch a concerted get-out-the-vote drive in the final days. Republicans will activate a version of the 72-hour voter turnout plan that paid huge dividends for Bush in 2004.

Obama is relying on a vast sea of "several million" volunteers to help turn out supporters, Plouffe said.

"Our goal here is to talk to every single person who we think is an Obama supporter, every single one, in these states," Plouffe said.

(Editing by Philip Barbara)