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Imagine this scenario: you’re in charge of an issue campaign opposing Pennsylvania Sen. Bob Casey’s (D) reelection. You got a bit of a late start, but still managed to raise a good chunk of change in the form of unlimited contributions through a Super PAC. Now you’re looking to buy some media time on WPXI—the NBC affiliate in Pittsburgh.

Media buyers like WPXI. It reaches a myriad of the state’s coal mining towns in Pennsylvania’s heavily unionized, but traditionally conservative, southwest. So you call up Sales Manager Cedric Thomas to ask about a flight of ads during the station’s news shows. Thomas doesn’t even bother with the rate card.

“Sold out,” he says. He sounds sympathetic, but says there’s nothing he can do.

Maybe there’s an episode of “The Mentalist” with an empty slot? Another primetime show, perhaps? “Afraid not,” Thomas says. “Once you’re sold out, you’re out.” Trying not to panic, you nonetheless feel a little like you’re at the supermarket shopping for hurricane supplies and finding only empty shelves where the batteries and water once were.

Get used to it, consultants and station managers warn. Airtime scarcity will be more of an issue in 2012 than ever before.

After the Supreme Court’s Citizens United decision, and with more than 300 Super PACs preparing to take to the airwaves against the backdrop of a presidential campaign that could very well pit the two best funded candidates in history against one another, the vital commodity of broadcast TV time could be hard to find.

“I believe this will be a record-setting year,” says Thomas. “In the battleground states, running out of inventory is a possibility.”

Does all of that sound just a bit far-fetched? Perhaps you’re thinking back to the not-so-distant past before McCain-Feingold and assuming this cycle will just resemble the soft money days?  

“I think it’s unclear because no one’s really bought yet for the cycle,” argues Democratic media strategist Jon Vogel, a former executive director of the Democratic Congressional Campaign Committee who isn’t yet convinced that 2012 will result in such a drastic run on airtime.

But consider this: By early February of this year, Super PACs had reported spending almost $50 million since January. Two of the largest Republican issue groups, Crossroads GPS and Americans for Prosperity, spent more than $8.7 million combined in just two weeks during the first month of the year. Their spots ran on broadcast and cable TV, from Albuquerque, N.M. to Wilkes-Barre, Pa.

President Obama’s campaign responded with an early barrage of its own, running more than 2,500 spots in 10 targeted media markets, according to an analysis by Wesleyan University and CMAG. 

“[The Obama campaign is] responding rather quickly to what the Super PACs are doing,” says Travis Ridout, a Washington State University professor who co-directs the Wesleyan/CMAG ad monitoring project. “It’s possible we’ll see ads running from now until Election Day.”

While the amount being spent in the GOP presidential primary is similar to what was dished out in 2008, there’s a big difference. Ads run by issue groups went from the low single digits in 2008 to totaling nearly half of all airings today, according to Ridout’s group. These are the folks who are going to be competing for the limited amount of broadcast airtime in battleground states come the fall.

President Obama and federal candidates like Casey don’t have to worry about an end-of-airtime scenario. The Federal Communications Commission ensures that all federal candidates can have access to the airwaves, even if they have to pay a premium during a busy campaign season. Down ballot candidates and issue groups, though, aren’t protected by the same most-favored-advertiser status. They can be bumped to less favorable ad times, have their ads dropped for other content, or told that there isn’t room for them on the airwaves.

This isn’t to suggest that groups with money to spend won’t be able to get on TV. Ad sales managers such as Thomas wield their inventory well. They can often shuffle their lineups to push spots for local car dealers and mattress wholesalers past Election Day to make room. Plus, there are always slots available during the daytime schedule, or on radio, cable or online. Even so, broadcast television remains the most efficient mass medium, and there’s only so much inventory stations can sell.

“The battleground states haven’t changed and there are competitive elections across the board,” says Paul Winn, a Republican media strategist. “Traditional inventory in these states is going to be sold out and we’re going to see the overflow go” to online, cable and radio.