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

By John Whitesides, Political Correspondent - Analysis

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.