After celebrating the benefits of robust ad spending in 2016, the U.S. media world may have to prepare for a hangover next year

Madison Avenue is seen paring back on the amount it spends in 2017 as advertisers ponder heavier investment in digital media and gauge the pulse of consumers under a mercurial new U.S. President, according to two of the ad sector’s biggest purchasers of commercials. While advertisers will spend more money overall, the amount of increase in their outlay from year to year is expected to narrow.

WPP’s GroupM, which represents marketers including Unilever and Anheuser Busch InBev, predicts ad spending in the United States will rise 3.1% in 2016 and a smaller 2.6% next year, citing weaker U.S. GDP growth and political uncertainty. Meanwhile, Interpublic Group’s Magna Global, which works for Coca-Cola and Chrysler, said it expects ad revenue collected by U.S. media owners to soar 6.9% in 2016, but to rise just 1.7% next year – while TV ad revenue will dip as part of the dynamic.

“The Trump victory has raised uncertainty,” said the GroupM report, released early Monday. “This means more hesitation in important decisions in the short-term, by people, governments and corporates. Advertising spend will likely be negatively impacted until there is clarity on policy.”

But there’s more going on. Behind the scenes, according to the Magna Global forecast, is new allocation to digital media. Magna predicts national TV ad sales in 2017 will fall by 0.9%, while ad dollars put behind social and digital video will rise 26%. Ad spending behind search is seen rising 14%.

Billions of dollars are at stake. Magna Global estimates advertisers spent $180 billion in the U.S. in 2016 and $493 billion around the world in the same year. GroupM estimates advertisers spent $178.8 billion in the U.S. in 2016 and $524.5 billion around the world in the same year. The media buying unit sees U.S. spending rising to $183.5 billion in 2017 and to $547.4 billion around the world.

To be sure, some of the pullback in growth was expected. The current year has been bolstered by both NBCUniversal’s recent Rio Olympics broadcasts as well as the extra advertising put into play in a heated U.S. presidential election. Any period following one that contains both events is generally expected to be one in which ad-spending activity moderates. But the pullback figures suggest the media sector will have to continue bracing for changes spurred by the rise of streaming video and mobile devices, and the effects those new technologies continue to have on traditional media including TV, print and radio.

The U.S. trends echo those in play around the globe. Magna Global forecasts media owners’ net global ad sales will slow down to 3.6% in 2017, compared with 5.7% predicted for this year. The company cited the lack of the Olympics and elections, as “economic and political uncertainty in several markets” due to Britain’s Brexit process, a new U.S. presidency, and general elections in both Germany and France. Digital media sales around the world are seen rising 13%, while linear TV ad sales will slip 0.1%, the company said.

Digital is fast becoming the engine that powers ad sales. GroupM expects 33% of global ad spending in 2017 to be devoted to digital media.  In 2017, digital will capture 77 cents per new dollar, GroupM said, compared with 17 cents for TV. In 2016, digital captured 72 cents of every new ad dollar, and TV 21 cents.

“Twenty years on from the internet becoming a measured ad medium, digital remains the engine of advertising growth and disruptor-in-chief of the entire marketing economy,”said Adam Smith, GroupM’s futures director.

Meanwhile, Magna Global predicts digital ad sales will become the number-one media category in 2017, reaching a market share of 40%, or $202 billion, compared to 36%, or $186 billion for linear TV ad sales. By 2021, digital will capture 50% of the overall market, or $299 billion. Compared with 33%, or $195 billion, for TV.