By Samuel P. Jacobs

WASHINGTON (Reuters) - In 2008, singer will.i.am provided Barack Obama's presidential campaign with music for its signature anthem, "Yes We Can." On Tuesday, at a rally for Obama in Columbus, Ohio, the performer chose to play something new: the theme song for "Sesame Street."

For Obama's supporters, already dismayed by the president's halting performance in last week's debate with Republican Mitt Romney, that change in tune is a new source for concern as they fret that a children's TV show has become a new backdrop for their candidate's campaign.

In a moment of tightening polls and climbing anxiety for Obama's supporters, the president's decision to grant Big Bird a starring role in his campaign this week has presented another reason to reach for the Alka-Seltzer.

After Romney named Big Bird as part of a promise to pull government funding for public television, Obama's campaign released a caustic new ad mocking Romney for thinking the character was a "big, yellow menace to our economy."

Since the debate, Obama has been piling on, joking about Romney's designs for the TV show at every campaign stop.

Conservatives have been crowing that the silly turn in the campaign diminishes the president.

"President Obama tried to give the bird to Mitt Romney—but wound up laying an egg," the New York Post wrote Wednesday.

Liberals point out that it was Romney who started the Big Bird mess. Still, the tactic may have led to a kind of role reversal for Obama and Romney. Throughout the summer, the Republican was criticized for lurching from one news cycle to the next, introducing attack lines that seemed to detract from his central message that Obama had stunted economic growth.

Now Obama, some Democrats fear, is seeking to revive his campaign with too light a jolt. They worry the president looks small by enlisting the eight-foot (2.4-metre) costume bird in his defense.

Romney's presidency would endanger more than a television character - if a beloved one - they say, and Obama's "Sesame Street" jabs belittle that peril.

"I'm not sure I understand why he is doing it," said Bill Galston, a former Bill Clinton adviser.

It got worse for the Democrats on Tuesday when the makers of Sesame Street asked them to pull the ad because they did not want Big Bird associated with politics.

The long-running U.S. children's television series, which first aired in 1969, uses a collection of puppets and costumed characters, puppeteer Jim Henson's Muppets, along with short films, humor and animation to promote early childhood education and creativity.

SEEKING THE AVIAN VOTE?

The more conspiratorial campaign watchers reckon maybe the president's team must know something Washington does not.

Perhaps, promising to save Big Bird is a winner among moms. A Pew Research Center survey released this week observed an 18-point swing in Romney's favor among likely women voters over the course of the last month.

Maybe the Obama folks think the only way to bandage the hurt caused by Obama's weak debate performance is with laughter.

The winking ad with its knowing use of irony could be a play for young voters, a nudge that says Obama is still the hip politician they knocked on doors for in 2008.

In a cloudy week where Democrats have formed a search party for silver linings, some hope the Big Bird ad is an attack line that merely hasn't reached its proper conclusion.

Before the campaign retires it, they hope Obama will link Romney's enthusiasm for canning the "Sesame Street" characters with a much larger statement about the former private equity executive's character.

Obama should talk about how Romney suggested PBS news host Jim Lehrer would lose his job too — and grinning while doing it, said Dick Harpootlian, Democratic party chair in South Carolina.

"There's nothing funny about firing anybody," said Harpootlian. "Why do you smile when you say you are going to fire somebody?"

The Obama campaign has said Big Bird was added to the campaign cast to shed doubt on Romney's seriousness as a candidate.

"When Mitt Romney was given the opportunity to lay out his plans for bringing down the deficit, he gave the same answer he has given dozens of times on the campaign trail, which was to cut funding for Big Bird," said Obama spokeswoman Jen Psaki.

"If that doesn't point out the lack of seriousness with his deficit reduction plan, I am not sure what does. The ad is an opportunity to highlight that."

Befitting a campaign that has turned toward toddler television, Romney's response has been, in effect, to say he is rubber and Obama glue.

"These are tough times with real serious issues," Romney said in Iowa Tuesday. "So you have to scratch your head when the president spends the last week talking about saving Big Bird."

(Editing By Alistair Bell, Todd Eastham and Eric Walsh)