Demand for the 15- and 30-second spots that automatically play before online videos are sold out in some markets or being auctioned off at record prices in others. - 

Tess Vigeland: We know presidential candidates spend a lot of money on TV ads. But in this Internet era, they're also advertising online. And they're finding that unless they reserve that space way ahead of time, they'll have to pay top dollar the closer we get to the election -- especially in swing states.

Marketplace's Nancy Marshall-Genzer has the story from Washington.

Nancy Marshall-Genzer: They’re the bane of every cat video lover’s existence: Those 30-second ads you’re forced to watch before your video begins. Now, especially if you’re in a swing state, you’ll see more political ads, like this one:

Political ad: Mitt Romney supports overturning Roe vs Wade. Click here to learn more.

A report from the online video advertising company Mixpo says demand for online video ad space will outstrip supply by 20 percent or more in 11 swing states. Matthew Feeney wrote the Mixpo report.   

Matthew Feeney: So when you see a large jump in demand and you match that with a short supply, obviously you’re going to see shortages.

And if candidates haven’t already locked in online ad space, they’re in for a big shock, according to Republican ad consultant Vincent Harris.

Vincent Harris: They’re going to wake up in mid-September and try to put money online and realize that they can’t. And they will essentially be shut out of advertising online in these specific states.

There will be some space left over. It’ll be auctioned off, and could get quite expensive, according to Jordan Lieberman of Campaign Grid.

Jordan Lieberman: Rates could double in certain places.

And we’re not just talking about swing states. Mixpo predicts shortages across 26 states this fall.

In Washington, I’m Nancy Marshall-Genzer for Marketplace.