Call it the upfront Armageddon. If there are four horsemen of the apocalypse for the advertising industry, they're all galloping toward this one.

Two of the three biggest advertisers in the world—Unilever and L'Oréal—have just called media reviews either globally (in the case of Unilever) or in the U.S. (for L'Oréal). Speculation is rising that the biggest global advertiser—Procter & Gamble Co.—will soon follow suit with its own U.S. media review.

Important reviews have overlapped the upfront before, but with such big accounts in flux, 2015 is definitely a critical year for media shops to show their worth. That puts the onus on contenders and defenders alike to drive the best deals in this summer's upfront.

Then there's the fear that advertisers, after years of veiled or direct threats to shift money from TV to digital, are following through. P&G and Unilever executives have said they're spending 30% to 35% of their U.S. media dollars on digital, with at least some of that yanked from TV. Last summer's upfront was weak, with digital rivals seeming to function as an undertow; soft ad prices since then suggest we could be in for a repeat.

Meanwhile TV sellers are fighting digital interlopers, telling agencies again and again about unviewable ads online and digital ad fraud. The last, indirect wild card is former media agency exec Jon Mandel's recent allegations that undisclosed rebates to media agencies are distorting where clients' campaigns appear. Pivotal Research analyst Brian Wieser last week downgraded WPP, Publicis Groupe, Omnicom and Interpublic on concern that, "rightly or wrongly," marketers will increase scrutiny of media deals and squeeze one of the agency industry's biggest sources of revenue.

These four factors all add up to this: sellers and buyers going into this TV upfront highly motivated to harden their bargaining positions.

"This upfront feels different," said Mr. Wieser. "The stakes are in some ways higher in that I don't think the noise around the death of TV has ever been higher in the 12 years since I've been paying attention."

To some extent, media agency reviews are a constant drumbeat, and Unilever's global reviews, historically done every three years, "you could almost set a clock by," Mr. Wieser said. But he couldn't recall a time when two of the three biggest global advertisers simultaneously held reviews at upfront time. Rising speculation around a P&G media review could add even more pressure.

P&G executives have increasingly talked about their efforts to improve performance of their ad investment, and have within the past year concluded reviews of the Canada media account and Latin America digital planning and buying account. The company has new leadership in the media area, with longtime top global marketing procurement executive Kim Kraus expanding her role to cover media, and North America Brand Director Kristine Decker having come on as the new top executive overseeing the U.S. media business. Plus, P&G hasn't done a review of the U.S. media account since 1997.

In an interview last month, P&G Global Brand Officer Marc Pritchard declined to address a review specifically. But he did say: "The industry is at a stage right now, and the world of brand building is at a stage right now, where we really have to step back and look at where we can create the most value … for consumers, for our brands and for shareholders. And therefore we're looking at everything, because we're at a real inflection point."

TV networks such as CBS, however, aren't being passive, telling marketers they know what's really good for them. In a presentation at the Advertising Research Foundation Re:Think 2015 conference last month, CBS Chief Research Officer David Poltrack pointed to studies it commissioned, based on Nielsen Catalina audience and consumption data, showing brands being hurt in return on investment as they moved funds to digital from TV.

In an interview, Mr. Poltrack said shifting money from TV to digital is "a really dangerous thing to do."

"It doesn't work," he said. "Even if digital is successful, by funding it out of your television budget … you have to subtract out the negative effect from reducing an effective and productive television schedule."

Come negotiation time, marketers will look to media agencies to navigate these questions for them—with some big accounts in the balance.

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CORRECTION: An earlier version of this story said that Mr. Poltrack referenced Unilever and Hellmann's in his ARF presentation. He did not.