Shares in WPP, the world's largest ad and public relations player, were recently down 2%, making it one of the biggest losers on the FTSE 100. And Paris-based Publicis was one of the worst performers on France's CAC 40, declining 1.7%.
WPP's and Publicis' competitors include New York-based Omnicom (OMC) , which whom Publicis tried to merge two years ago, and Interpublic Group (IPG) . WPP derives 35% of its revenue from the U.S., while Publicis makes half of its sales from that market, according to Factset.
Analysts said declining U.S. advertising spending as well as a weaker dollar could hurt WPP and Publicis in the short term.
"U.S. advertising spending could see a short, sharp shock," Liberum Capital analysts noted before Trump's victory was confirmed. "A lot of the high margin work the agencies receive in Q4 could be pulled in the event of a Trump victory. This is obviously a short-term risk but one that needs to be considered."
WPP proved itself a winner within the sector when the U.K. voted to leave the Europe Union on June 23. Even as the pound plunged following the Brexit vote, the company's shares rose thanks to its geographical spread, which includes China, Germany, France, as well as the U.S.
WPP shares have rallied nearly 10% since the Brexit vote.
WPP's revenue from North America grew 3.8% in first nine months. WPP owns Ogilvy & Mather Worldwide, Y&R, Grey and J. Walter Thompson.
For Publicis, the owner of Saatchi & Saatchi, North America accounted for about 54% of third-quarter revenue, Europe for 28%, Asia 11%, Latin America 3.8%, and the Middle East and Africa nearly 3%.
In the long term, Jefferies suggested ad energies will benefit from Trump's plan to cut the corporate tax rate to 15% from 35%.
"This would be beneficial for agencies all else being equal," Jefferies analysts noted. "These savings would provide a boost for spending, which is likely to lead to increased investment in advertising and innovation, a positive for the advertising agencies."
Among other sector players, shares in U.K.-based Creston recently dropped 3.9%, while France's Havas shares were down 0.5%.