Big Bird, the tall yellow Muppet beloved by children across the world, has become a key figure in the US presidential campaign, and is featuring in a new advertisement issued by US President Barack Obama.

Obama on Tuesday put out a commercial mocking his opponent, Mitt Romney, for saying he would cut funding for PBS, the public broadcaster behind the TV show Sesame Street, in which Big Bird appears.

"Romney knows it's not Wall Street you have to worry about, it's Sesame Street," the Obama ad said, jokingly describing Big Bird as the "evil genius" towering over financial felons like Ken Lay and Bernie Madoff.

Obama's Big Bird TV advertisement

"Mitt Romney. Taking on our enemies, no matter where they nest," the announcer of the commercial says, pressing home Obama's contention that Romney, a former venture capitalist, would let Wall Street run wild.

Analysts have said that cutting the million of dollars a year that the broadcaster receives from Congress will do little to help tackle the country's $15 trillion debt.

Romney responded by feigning puzzlement about why Obama would dwell on such a triviality four weeks before election day.

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

Romney said in a debate in Denver last week that "I like PBS. I like Big Bird," but nevertheless pledged to cut the subsidy for the station as part of spending reductions he plans if he is elected president in November.

Obama did not pick up on the comment during his highly criticised debate performance, but has since made Big Bird, who has helped many children in the US and around the world to read, as a feature of his stump speech.

'Fight for Big Bird'

Jen Psaki, the president's spokeswoman, told reporters aboard Air Force One on Tuesday: "There is only one candidate in this race who is going to fight for Big Bird and Elmo and he is riding on this plane."

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Psaki also made what the Obama campaign sees as a serious point about Romney's failure to provide concrete examples in the debate about what he would do to trim the budget deficit.

"When Mitt Romney was given the opportunity to lay out how he would address the deficit ... his first offering was to cut the funding for Big Bird," she said.

Sesame Workshop, the maker of Sesame Street, said meanwhile it was a non-partisan, non-profit organisations, and has asked that the ad, which features footage of Big Bird, be removed.

"We do not endorse candidates or participate in political campaigns," the organisation said in a statement.

"We have approved no campaign ads, and as is our general practice, have requested that the ad be taken down."

Psaki said the Obama campaign had received the request and was reviewing its options on the commercial, which is running on national cable stations.