By Diane Bartz and Olivia Oran

WASHINGTON/NEW YORK (Reuters) - U.S. corporate dealmakers were likely to put major merger plans on hold as they assess whether U.S. President-elect Donald Trump will follow through on his populist promises and a threat to block AT&T Inc's <T.N> purchase of Time Warner Inc <TWX.N>, or act more like traditional business-friendly Republican administrations.

Trump's rhetoric and the personal nature of the campaign, which included little discussion of policy, left many uncertain about the new U.S. leader's plans, including how his administration will handle mega-mergers.

Wall Street braced for a drop in deals, with Goldman Sachs on Wednesday projecting a 20 to 30 percent downside for earnings of banks that focus on merger and acquisition advice, and Jefferies saying that uncertainty about Trump's policy on trade, healthcare, taxes and energy could hamper underwriting activity and M&A globally.

"I think a lot of deals will hit the pause button for a bit until we get some clarity on whether President Trump will moderate or be as disruptive as some expect," said a senior Wall Street banker who asked not to be named because he was not authorized to speak with the media.

"It's going to be a tough environment for everything until we see how [Trump] behaves as a leader," the banker added.

Trump said in October that AT&T's proposed $85 billion acquisition of the owner of HBO, CNN and the Warner Bros film studio was an example of a "power structure" rigged against him and voters, and that he would block a deal.

The gap between Time Warner shares and the implied value of AT&T’s cash and stock bid was over 23 percent in afternoon trading on Wednesday, compared to around 22 percent at Tuesday's close, indicating greater investor skepticism that the companies will be able to complete the transaction.

Still, some investors believed the man who considers himself business friendly would take a more moderate tone than in the campaign once he assumes office, as he did on Tuesday night in his acceptance speech.

"We think Trump will be pretty good for merger and acquisition activity. As a general proposition, he is pro-business and pro-free market," said Roy Behren, portfolio manager at Westchester Capital Management.

Other big pending U.S. deals also did not see sharp changes in their spreads on Wednesday morning, and the spreads of three pharmaceutical or health care-related deals that have encountered antitrust troubles, Aetna <AET.N>-Humana <HUM.N>, Anthem <ANTM.N>-Cigna <CI.N> and Walgreens <WBA.O>-Rite Aid <RAD.N>, actually narrowed, signaling investors may think they are more likely to close under a Trump administration.

The president does not directly decide if a merger is illegal under antitrust law and the job is done by the U.S. Justice Department or Federal Trade Commission, which divide up the work of assessing mergers. If one of the agencies decides to stop a deal, it must convince a judge to agree.

AT&T Chief Financial Officer John Stephens on Wednesday said his company was looking forward to working with Trump and "optimistic" regulators would approve the deal.

Trump's policies and discussions "about infrastructure investment, economic development, and American innovation all fit right in with AT&T's goals," Stephens said at the Wells Fargo technology, media and telecoms conference in New York.

Time Warner's shares were last down 1 percent to $86.71, after trading as low as $85.60, while AT&T shares were up just under 1 percent at $37.24. The Dow and S&P 500 were both over 1 percent higher in late afternoon trading.

The election results mean "increased risks" for the AT&T-Time Warner deal, Angelo Zino, analyst at CFRA Research, said.

"At the very least, there are going to be individuals put in place (by a Trump administration) that are going to make the deal a lot more challenging to complete," he added.

Trump's protectionist stance also raises the risk that some foreign corporations, including from China, may face higher hurdles in trying to take over American companies, dealmakers said.

"If you were thinking about doing a cross-border deal six months ago you weren't considering things like potential trade barriers, protectionism and tariffs. These are things you have to at least develop a view on and factor into the risk assessment of doing an overseas deal now," said Johs Worsoe, MUFG’s head of investment banking & markets in the Americas.

(Reporting by Malathi Nayak and Diane Bartz in Washington, Michael Erman in New York, Aishwarya Venugopal and Swetha Gopinath in Bengaluru,; Writing by Peter Henderson, Editing by Soyoung Kim and Meredith Mazzilli)