For most of this year, top television execs have nervously wondered whether the economic slowdown would put a crimp in the generally robust national TV advertising market.

As the magnitude of the meltdown in world financial markets came into sharper focus over the past few weeks, the question changed from “Will it?” to “How bad will it be?”

Sales execs at the Big Four networks and major cablers say there’s no longer any doubt ad sales will take a hit next year, if not sooner. CBS Corp. and Viacom acknowledged as much Oct. 10, when both were forced to significantly reduce their earnings forecasts for the third quarter. The turmoil, coupled with the near-certainty of a recession on the horizon, is bad news for anyone in the business of selling advertising.

“Certainly, as a seller we’re reacting to the marketplace,” says Mike Shaw, ABC’s prexy of sales and marketing. “It’s put a premium on the conversations we’re having (with advertisers) regarding those hard-fought marketing dollars.”

As the stock indices go through more gyrations than the hoofers on “Dancing With the Stars,” ad biz execs say they’re bracing for a lot of coin to evaporate.

“I think there will be a fair amount of money that advertisers take out of January, February and March, but it’s hard to predict how much,” says media buyer Andy Donchin, director of national broadcast for Carat North America.

Ad giant Zenith Optimedia has cut its growth forecast for all U.S. advertising expenditures for this year almost in half, to 1.8% from the 3.5% it predicted in June. For next year, Zenith foresees an anemic 0.9% growth rate for U.S. ad spending.

Longtime ad biz analyst Jack Myer last week predicted that total domestic ad expenditures will fall 4% next year, after ebbing 1.3% this year. He projects online ad spending in 2009 will be essentially even with this year’s at a 13.5% growth rate.

The carnage has already started at the local level. Local broadcast TV stations are facing the grimmest market in years, with key advertisers like auto dealers and local banks barely able to do business. Analyst Jeffrey Logsdon of BMO Capital markets lowered its price target on News Corp., among other media stocks, last week, citing the weakness in local ad markets that are already posting declines of as much as 15%-20% compared to this time last year. UBS projected in a research note last week that News Corp.’s O&O stations could see revenues sink as much as 25% next year.

CBS, in its earnings warning, said it would take a $14 billion noncash writedown in the third quarter to reflect the loss in value of its 29 TV and 140 radio stations.

The chaos in the financial markets during the past month has already taken a heavy toll on big-spending TV advertisers, particularly financial institutions, automakers and homeowner-centric retailers such as Home Depot.

Sports programming in particular is heavily dependent on ad dollars from automakers, banking and insurance firms. Almost overnight, there’s been a massive contraction in those categories with the shotgun sales of once big-spending brands like Merrill Lynch, Wachovia and Washington Mutual. Rumors of a merger among two of the Big Three domestic automakers persist.

Sales chiefs at the major nets are quick to point out that national television has historically ridden out recessionary periods better than most other ad-dependent media. Indeed, there’s much speculation that digital advertising will be walloped if marketers do slash budgets.

“When times get tough, people cut back on the new and the experimental and fund the tried and true that they know works,” says Jon Nesvig, Fox Broadcasting’s prexy of sales.

Still, Hollywood is already feeling the effects of this belt-tightening as studios keep a tight rein on production budgets, even for successful skeins. Producers are being told to get creative and forget about extraordinary location shoots and lavish f/x work that might have been OK’d a year ago.

Among the shows feeling the budgetary constraints, according to anecdotal grousing among producers, are CW tentpole “Smallville,” Fox’s action-oriented “Terminator: The Sarah Connor Chronicles” and “Prison Break”; and NBC’s “Heroes.”

If the ratings declines suffered during the past few seasons weren’t already pushing spot prices down, the tightening of marketing budgets at the biggest advertisers certainly will.

The top buyers of advertising on TV — including Procter & Gamble, AT&T, General Motors, Anheuser-Busch and Coca-Cola — have gone into cost-cutting mode, with a significant portion of the dollars saved coming from marketing budgets. GM got showbiz’s attention when it backed out of its traditional blurb presence in last month’s Emmycast and next year’s Oscarcast.

Execs at the major nets say they expect to get through the rest of the year without a tidal wave of pullouts by advertisers, if only because fourth-quarter marketing dollars have been earmarked for some time.

“The wild card is what happens next year,” says a top sales exec for a showbiz conglom.

The nervousness began back in the spring.

For the first time in years, the top sellers didn’t have a strong read on how the top buyers intended to play the upfront market.

“It could’ve gone either way, and it was nerve-wracking,” admits a Big Four sales vet.

In fact, upfront sales turned out to be healthy, far exceeding worst-case scenario predictions. The Big Four networks had to sell a little bit more inventory than in years past to stay even with last year’s $9 billion haul, but marketers didn’t seem too spooked about economic conditions.

Another reason demand in the upfront was so strong was that the scatter market, or short-term sales of remaining ad inventory, had been incredibly robust for the previous 12 months or so. Advertisers were eager to lock in the guaranteed lower rates offered by the nets to buyers making longer-term upfront commitments.

But upfront sales are not done deals. They’re holds on ad time that buyers have the option of ordering or releasing throughout the season as airdates draw nearer.

As economic indicators continued to head south, there was much gnashing of teeth over the summer as to whether most of those holds would turn into firm orders. If not, the scatter market would be flooded with available spots and pricing there would flatten out considerably.

One month into the 2008-09 season, the feared stampede of cancellations hasn’t materialized, but the scatter market has slowed considerably, both in volume and pricing. And pricing in the scatter market is a key yardstick for the immediate health of the ad market. So far, scatter prices are hovering around upfront levels; execs concede that scatter pricing will probably drop further in the first half of next year.

Industry execs note that some of the bigger cablers — USA, TNT, ESPN, FX, etc. — are doing as well or better proportionately in scatter sales as their broadcast counterparts in recent weeks. It’s a sign that the more established cablers may weather the storm better because advertisers see them as offering more bang for the buck in demos.

On the flipside, newer cable and satellite channels targeting niche auds may well get killed in an ad downturn, along with other new-media platforms. However, execs at Google and other netcos have argued in recent weeks that broadband video and online lifestyle-entertainment sources could be become more popular if hard times force more people to cancel their cable or satellite services.

Daniel Frankel and Marc Graser contributed to this report.