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The mushrooming of issue advertisers this cycle, combined with the presidential race is expected to put a severe strain on ad inventory. How bad could things get? In addition to shortages of coveted broadcast TV ad time, we could see online inventory get gobbled up, too. "Scarcity will definitely be an issue," says Sean Harrison, an account executive with Google's elections and politics team.

Buying for the fall has already started and inventory on high-trafficked sites is going fast, says Harrison, speaking during C&E’s CampaignTech conference Friday. "It's a pretty tough market -- we're not even in May yet and campaigns are already thinking so forward."

In the corporate world, advertisers are starting their campaigns earlier to generate buzz around their products. The week before the Super Bowl in February, for instance, Volkswagen released the full version its golden retriever-Darth Vader commercial. That same tactic is being adopted by political advertisers, Harrison says. "The earlier you run your ad the more affect it's going to have." 

Mat Harris, a product manager at the San Francisco-based Sprout, disagreed with Harrison on scarcity. "Inventory is only an issue if you care where your ad runs," he says.

Some advertisers insist their spot appears on, say, the New York Times' mobile site, he says. "You can find the audience in other places. You have to have improvisation as part of your strategy."

When you do run ads in this soon-to-be crowded space, it pays dividends for a candidate to make it as personal as possible. "The more personalized you can make it, it cuts through the clutter," says Sean Duggan, a vice president of advertising with Pandora Internet Radio.