This year, 6 types of voters will decide the presidential election

— -- All voters are not created equal.

Democratic presidential candidate Barack Obama leads Republican John McCain in the latest USA TODAY/Gallup Poll by only single digits among registered voters, 48%-42%, at the edge of the survey's margin of error.

However, an analysis of how Americans view the election — whether they think it matters and how strongly they're committed to a candidate — shows a more lopsided contest.

USA TODAY used results from a national survey of 1,625 adults to sort the electorate into six broad groups based on their level of engagement and enthusiasm about the election.

Obama dominates the two most energized groups of voters, 44% of the electorate combined, who are focused on a range of issues and say they won't change their choice of candidate between now and November. McCain's strongholds are two groups of voters at the other end of the spectrum, 28% of the electorate in all, who are skeptical that the election results will make any difference in their lives and are less enthusiastic about voting than usual.

A cluster of more upbeat GOP-leaning voters remains in the middle and up for grabs.

In all, 67% of Obama supporters say they're more excited than usual about voting, compared with 31% of McCain backers. A 54% majority of McCain voters report being less excited than usual.

Political strategists who have run national campaigns say that enthusiasm gap underscores an uphill climb ahead for McCain — and reflects a risk for Obama. Everything is harder when your backers are downbeat, says Tony Fabrizio, the pollster for Republican Bob Dole's 1996 presidential campaign, which struggled to generate voter excitement.

"It is very difficult to raise money if there's not a level of enthusiasm," he says. "It is very difficult to put people out on the street. It's very difficult to have a national organization, or to get people to rallies and events." He's seen a "stark contrast" between the ability of McCain and Obama to draw big crowds and raise money.

If there is a silver lining for McCain, it's this: At a time President Bush's approval rating is 28% and fewer Americans identify themselves with the GOP than at any time in decades, those backing the Arizona senator are likely to stick with him. What's more, they tend to be older people who reliably vote.

Obama, in contrast, has inspired enormous enthusiasm among younger voters, who in the past have been less likely to show up at the polls.

Their participation will help reshape the electoral map if they stay engaged, says Joe Trippi, a strategist for Democrat Howard Dean's youth-driven presidential bid four years ago. But "all that energy sometimes is fragile if you make a big mistake," he cautions. "When you have a bunch of people who are very enthusiastic, it's easier to disappoint them."

Even so, Trippi adds, "I'd rather have the energized people."

The USA TODAY analysis is based on four questions from the poll, taken June 15-19:

• Compared to previous elections, are you more or less enthusiastic than usual about voting?

• How much difference do you think the election result will make to you and your family?

• Do you have a favorable or unfavorable opinion of John McCain and of Barack Obama?

• Are you certain now whom you will vote for, or do you think you may change your mind?

Here's what the analysis found.

1. True believers:30% of the electorate

Nearly one-third of those surveyed could be called the true believers of this campaign.