Unlike political candidates, Google (NSDQ: GOOG) and other online ad providers can receive an unlimited amount of campaign cash. While they won’t receive the kind of ad money that TV and radio will attract in this election season, Sen. Barack Obama is voting for Google in the form of some of its ad spending. The Democratic presidential hopeful has contributed about $3.5 million in campaign spending to online performance-based ad networks this year, with $2.8 million—roughly 80 percent—going to Google, ClickZ reported, citing Federal Election Commission filings. Microsoft (NSDQ: MSFT) received a much smaller portion of Obama ad spend, about $73,000, probably through a combination of display ads on MSN and sponsored search.

As the piece makes clear, political campaign ad spend could reach $3 billion this year, and online will likely remain a small part of that. Since most political sites tend to reflect a particular viewpoint, the use of political ads as a tool of persuasion would be considered largely unnecessary.  As the filings show, Obama’s campaign did make some small direct buys on specific sites, including Politico ($36,000), CNN.com ($24,000), and Gothamist ($2,800).

The interesting thing about the ad buys from the Obama For America campaign is that it reflects the kind of spending pattern evinced by traditional marketers, with a particular focus on cost-per-action ads. Aside from Google and Yahoo (NSDQ: YHOO) search ads, the Obama campaign has spread its funds across AOL (NYSE: TWX) Platform-A’s contextual unit Quigo and Microsoft’s DrivePM as well as Specific Media, and Pulse360. For Yahoo, Obama divided spending between its search and display ads fairly evenly, with $180,000 seeming to go to the former and $172,000 to the latter. ClickZ tries to ferret out where the campaign’s spending on Google went. It found that $18,000 was paid to “Google Image Domain,” which likely refers to the AdSense display network. Another $2.8 million went simply to “Google Inc.” As for social nets, Obama chose to spend $47,000 on Facebook, versus $11,500 on MySpace.

• @ EconSM: Election ‘08 And The Rise Of Voter-Generated Content