by: Craig Timberg | The Washington Post

Don’t expect any ads for President Obama across the top of Prospect.org, the online incarnation of liberal monthly magazine the American Prospect. However warmly editors there may feel about him, it is blocking ads from his campaign as part of a pricing dispute that has pitted many political Web sites — on both the left and right — against their ideological allies.

The standoff is an unintended consequence of a broad shift in political advertising this campaign season. More money is going into online ads than ever before, with estimates topping $100 million. But much of this bounty is being distributed through advertising exchanges, such as AOL’s Advertising.com or Google AdSense, that serve as middlemen, bypassing the direct buys that long have been key sources of revenue for Web publications.