There’s a long-held belief that sports programming is immune to economic downturns, a theory that’s proving to be less true each day the ad economy slides.

But one sporting event is holding up remarkably well, all things considered: the Super Bowl, set to air Feb. 1. NBC says it has sold about 90 percent of units at an asking price of $3 million for each of 67 commercials, a substantial windfall in a down economy.

Last year’s Super Bowl on Fox generated $196 million, according to Nielsen Monitor-Plus. Thirty-second spots went for about $2.7 million.

The Super Bowl most years is the most-watched event on TV, with about 40 percent of homes tuning in.

“You have to be pretty bullish on the Super Bowl because it’s still going to be delivering the audience and perhaps more so than in the past,” says David Carter, principal at Sports Business Group, a consultancy.

“It might be a real rallying point for fans and casual viewers because there is a lot of fatigue with news of all the financial problems. If I’m an advertiser, I might be looking at those spots and thinking the timing might not be ideal but that’s where my customers are going to be.”

Still, even the Super Bowl is feeling the effects of the worst ad economy in memory.

Sports analysts say NBC will eventually sell all its units but not without a struggle. It’s likely NBC will go back to some advertisers that have already booked time to sell an additional spot at a discounted price, perhaps bundled in with no-cost units to run at a later time.

And NBC will probably see a falloff in the amount of ad revenue it generates for its pre-game coverage.

But NBC mostly lucked out with the Super Bowl because long-term advertisers like Anheuser-Busch typically book time months in advance. NBC was selling units before the worst of the economic slump.

“The majority of the Super Bowl spots were sold prior to the fall, somewhat insulating NBC from the steepness of the economic downturn,” says Scott Becher, president of consulting firm Sports & Sponsorships. “Listen, if NBC gets close to an inventory sellout at $3 million per spot, they’ve done a fantastic job.”

Moreover, the appeal of the Super Bowl to advertisers remains strong.

The sheer size of the audience is unrivaled, and it generates a slew of press coverage for advertisers.

Last year, the Super Bowl pulled a 43 household rating when the New York Giants defeated the New England Patriots. That was the game’s best rating since 1999, helped by two teams from big media markets with huge fan bases.

This year, there’s a chance the Giants will be in the Super Bowl again. But smaller-market teams like the Carolina Panthers may be playing. The NFC and AFC divisional playoffs are this weekend.

But ratings aren’t likely to suffer, says sports consultant Neal Pilson, particularly with many cash-strapped consumers staying home rather than paying for outside entertainment.

“People are staying home, they’re not traveling, they’re not taking advantage of high-end entertainment and they’re not shopping,” he says. “You know what they’re doing? They’re staying home watching television, which I think is a pretty good indication we’ll see good ratings for the Super Bowl.”