WASHINGTON - Rush Limbaugh once coyly and repeatedly used the word "spade" when discussing Barack Obama and frequently called him an "affirmative action candidate" as he ran for president.

But a scathing joke told about Limbaugh by comedian Wanda Sykes at this weekend's White House Correspondents' Dinner has had conservatives in full lather for days, with much of their anger directed at the president for laughing at the Sykes smackdown.

Sykes took aim at Limbaugh's stated desire for Obama to fail, calling it treasonous and comparing the talk radio megastar - famous for battling an addiction to painkillers - to 9-11 mastermind Osama bin Laden.

"Maybe Rush Limbaugh was the 20th hijacker but he was so strung out on Oxycontin he missed his flight," Sykes quipped, adding she hoped Limbaugh's "kidneys fail."

Obama laughed at the joke, took a sip of water and continued to chuckle.

Bad move, said Canadian David Frum, a prominent conservative pundit and one of Limbaugh's harshest critics.

"The central part of the story is not the comedian but the president," Frum said Monday.

"One of the things I would have imagined, as an experienced TV performer like Barack Obama would know, is that you don't have to laugh at every joke. You know there's going to be a reaction shot ... if he had sat there and not laughed, not smiled at that, the story would be about her, not him."

Conservatives far and wide seem to agree.

"That's way, way beyond reasoned debate or comedy and Obama's reaction to it was astonishing," wrote the London Daily Telegraph's Toby Harnden, who attended the dinner.

"Obama laughing when someone wishes Limbaugh dead? Hard to take from the man who promised a new era of civility and elevated debate in Washington."

Democrats in attendance at the event have pooh-poohed the controversy, reminding outraged conservatives that Limbaugh has made a career out of making incendiary statements. And Sykes, they pointed out, is a comedian known for her no-holds-barred humour.

Even a conservative defended the joke on Monday.

"I still hate the post-game whining about how 'inappropriate' Sykes's remarks were," Tucker Carlson, a conservative pundit on MSNBC, said during a live chat on the Washington Post website.

"In fact, I hate the word 'inappropriate' ... I'm for humour, inappropriate or not."

Nonetheless White House press secretary Robert Gibbs made efforts Monday to distance the administration from Sykes's comments.

"I don't know how the guests get booked," Gibbs said, adding that he hadn't "talked specifically" with Obama about Sykes's crack.

"I think there are a lot of topics that are better left for serious reflection rather than comedy. I think there's no doubt 9-11 is part of that."

It's certainly not the first time the correspondents' dinner has featured a controversial performance by a comedian that has left D.C. reeling in the days to follow.

In 2006, Stephen Colbert, in character as his buffoonish right-wing television mouthpiece, delivered a searing rebuke of George W. Bush and the media's softball coverage of him.

New York Times columnist Frank Rich called Colbert's after-dinner speech a "cultural primary," christening it the "defining moment" of the U.S. mid-term elections that year.

Like Colbert, Sykes was directing her ire at an easy target - Limbaugh, after all, has made a litany of racist, sexist and otherwise offensive comments over the course of his career. He once infamously accused Canadian actor Michael J. Fox of exaggerating the effects of his Parkinson's disease.

Limbaugh's attacks on Obama have frequently centred on his race, with numerous suggestions that he only got elected because he's black.

"Obama is holding his own against both of them (Bill and Hillary Clinton), doing more than his share of the spadework, maybe even gaining ground at the moment, using not only the spade, ladies and gentlemen," Limbaugh once said.

"But when he finishes with the spade in the garden of corruption planted by the Clintons, he turns to the hoe."

Might Obama have laughed at Sykes's jokes because he's been nursing a grudge?

It doesn't matter, says Frum, who maligned Limbaugh's brand of divisive conservatism in a recent Newsweek magazine cover story.

"Limbaugh's not president," Frum said.

"And his words have exacted a price for him. That's the reason why Limbaugh is a marginal person - he's got a large audience, but he's got a large non-audience as well."

But Obama's appeal, Frum says, is his ability to rise about the fray.

"Vindictiveness is never an attractive quality," he said.

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