WASHINGTON – That smiling guy walking down the street? Odds are he's a Barack Obama backer. The grouchy looking one? Don't ask, and don't necessarily count on him to vote next week, either.

More John McCain supporters feel glum about the presidential campaign while more of Obama's are charged up over it, according to an Associated Press-Yahoo News poll released Saturday.

The survey shows McCain backers have become increasingly upset in recent weeks, a period that has seen Obama take a firm lead in many polls. One expert says the contrasting moods could affect how likely the two candidates' supporters are to vote on Election Day, possibly dampening McCain's turnout while boosting Obama's.

While 43 percent of the Democrat Obama's backers said they are excited over the campaign, just 13 percent of McCain's said so, according to the survey of adults, conducted by Knowledge Networks. Six in 10 Obama supporters said the race interests them, compared to four in 10 backing McCain, the Republican senator from Arizona.

On the flip side, 52 percent of McCain supporters said the campaign has left them frustrated, compared to 30 percent of Obama's. A quarter of McCain backers say they feel helpless, double the rate of those preferring Obama, the Illinois senator.

More McCain supporters also feel angry and bored, while Obama's are likelier to say they are proud and hopeful.

All of this is a bad sign for McCain, according to George E. Marcus, a political scientist from Williams College who has studied the role emotion plays in politics. Negative feelings about a campaign can discourage voters by making them less likely to go through what can be a painful process: Voting for someone who will lose.

"If I'm getting my head handed to me by a tennis player, my brain is saying, 'Do I want a second match? No,'" Marcus said. "Why do something that's going to lead to failure?"

Marcus said such emotions can be overcome by outside events, such as a campaign or neighbor urging a person to vote. There's also the danger exuberant Obama backers might decide not to vote because of overconfidence. The Obama and McCain organizations combined have spent hundreds of millions of dollars for those very reasons.

Obama leads McCain among likely voters in the AP-Yahoo News poll, 51 percent to 43 percent.

Supporters of McCain cite a dislike for Obama, dissatisfaction with the campaign's tone and frustration with how news organizations have treated their candidate.

"Flat disgusted, how's that?" said Billie Hart, 80, a Houston Republican backing McCain. "Because that's the way I feel about it. I don't like the individual."

Many Democrats say they're energized by a candidate they perceive as different from most politicians and who can make a real difference.

"Elections have always been so ho-hum," said Kathleen Rockwell, 61, an Obama supporter from Redmond, Wash. This time, "I feel connected. And that feels good."