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One of Madison Avenue’s most influential media-buying executives is stepping down from her post, just weeks before U.S. TV networks are set to start making pitches for ad dollars as part of the industry’s annual “upfront” market.

Christine Merrifeld, who has influenced major decisions by Procter & Gamble, Honda, Mondelez International and Coca-Cola, among others, at MediaVest, the ad buying shop owned by Publicis Groupe, is leaving the firm, the company said Friday. Merrifield has held the title since 2012, when she replaced Donna Speciale, an influential ad buyer who moved to run advertising sales at Time Warner’s Turner unit.

During her tenure, Coca-Cola stopped buying ads during ABC’s broadcast of the Oscars, where it had been a charter advertiser for some time and, more recently, ended its prominent sponsorship of “American Idol” on Fox.

Merrifield will be replaced by Melissa Shapiro, who had been executive vice president of investment at MediaVest and who has also worked at WPP Group’s MindShare, another ad-buying agency.

Only a handful of executives have as much influence over ad spending as Merrifield – and now Shapiro. These buyers can help guide big advertisers to invest more heavily in new media, such as mobile or social, and less in traditional, like television, and the ability to win their influence could play a major role in how the next upfront, when TV networks try to sell the bulk of their ad inventory, plays out in 2015.