At this time four years ago, Democrats were immersed in an intra-party fight over former President Barack Obama's choice to replace Ben Bernanke as chairman of the Federal Reserve.

President Trump has yet to seriously begin the nomination process for his own pick for the top economic policy-making post in the world, but a major battle is coming, one that has the potential to unnerve investors.

Fed Chairwoman Janet Yellen, whose term ends in February, is not the only official who Trump must replace. He also will likely have to appoint a new vice chairman and three other governors at the seven-member Board of Governors. He has nominated one candidate, Randal Quarles, for the post of vice chairman of supervision.

"We're talking about a change at the helm of the central bank that's probably the largest change in 1936," said Diane Swonk, CEO of DS Economics. "That brings a tremendous amount of uncertainty."

Four years ago, the fight pitted Obama, who was inclined to nominate his former adviser Larry Summers, against liberals such as Sen. Elizabeth Warren, who saw Summers as too friendly to Wall Street and wanted to steer him toward Yellen.

By mid-September, Summers withdrew his name amid pressure from Senate Democrats, clearing the way for Yellen and putting an end to the ideological sniping.

Trump can count on Democratic opposition to his candidate for chairman, given that he wants someone who can help carry out his deregulatory agenda. Yet his stated preference for someone who favors looser monetary policy also could cause trouble on the Right.

Relatively few potential central bankers fit that profile. And the two people thought to be the leading the running are currently on Trump's bad side.

Gary Cohn, Trump's National Economic Council director, was rated as the most likely pick to take over at the Fed, according to a survey of economists polled by the National Association of Business Economists in late July and early August. Trump said explicitly in a July interview with the Wall Street Journal that he was considering nominating Cohn for the role.

But some observers think that Cohn, who is Jewish, may have damaged his chances by voicing criticism of Trump's comments on white supremacism, neo-Nazis, and the KKK.

Cohn's public rebuke "might be enough to cause a president who places a high value on personal loyalty to rethink his intentions," said Terry Sheehan, an economic analyst for Stone McCarthy Research Associates.

Trump also has said that he would consider reappointing Yellen, on the basis that she is likely not to favor higher interest rates. "I'd like to see rates stay low. She's historically been a low-interest-rate person," he said in the same interview.

But Yellen may have hurt her cause with a high-profile speech at an August central banking conference in Jackson Hole, Wyo., in which she rejected the idea that Obama's rules on banks have slowed economic growth and recommended against major changes. That theory undergirds the Republican effort to relax the rules.

Trump and his advisers have called for removing much of the 2010 Dodd-Frank reform law, and, failing that, lessening the burdens of regulations as much as possible through administrative action. In a trip to Kentucky last month, Treasury Secretary Steven Mnuchin suggested that 80 percent of the administration's agenda could be carried out through the agencies, without Congress' help.

A Fed chief resistant to that agenda could easily stymie it, however.

Yellen's tough line on financial regulations underscores the difficulty that Trump could face in finding a candidate who would share his deregulatory agenda but who senators and business groups also would trust with major influence over the world economy.

"I think he would have trouble finding someone likely that, someone who is both credible to markets and who could survive the nominations process," Sheehan said. "Kind of a hard sell."

Usually, the political considerations that go into other nominations are less relevant for the Fed chairman. Instead, the primary consideration for presidents is the desire to have someone at the central bank who could minimize the likelihood of a financial crisis.

Liza Wright, a managing partner at the executive search firm Lochlin Partners who helped President George W. Bush in his search for a successor to Alan Greenspan, said that only a handful of people met the basic criteria to be entrusted with the world's largest economy.

"We had to make sure that anyone we were going to announce for such a huge job was going to be someone that was going to be instantly, based on their credentials and experience, respected," Wright said.

That is why recent Fed chiefs have had backgrounds in both academia and government. Most presidents simply want to play defense and pick someone who won't get them in trouble, said Action Economics' Mike Englund.

Trump could be different, however, and there is a "possibility that a name pops up out of the blue," Englund said.

If Trump were to look outside the relatively small circle of people with economics PhDs and experience at the Fed, he would face unique difficulties rounding up Republican votes. A businessman such as Cohn, for example, would see scrutiny placed on his business dealings. In Cohn's case, that would include the investment and banking decisions he made during the financial collapse.

And he also could face trouble winning support from senators who have no basis on which to judge his thinking on monetary policy. "That is clearly the downside for him, it's not clear what his opinions are going to be," Englund said.

Trump has one major advantage that past presidents didn't have: The filibuster no longer applies to Fed nominees, meaning that Democrats won't necessarily be able to stop candidates. Yet, the calendar is a bigger problem.

Even if Trump is able to line up candidates and build support for them in the Senate, the newly reshaped board he establishes will be responsible for the Fed's ongoing campaign to normalize interest rates and withdraw stimulus almost immediately, with little phase-in. While the Fed is heavily influenced by career staffers and inertia, said Swonk, that's a "very short lead time."