Then, in a reference to the Fed’s slow shrinking of its huge portfolio of bonds, which it had bought as part of its campaign in 2008 to rescue the economy, Mr. Trump added, “Quantitative tightening was a killer.”

Mr. Powell is a lawyer, financier and Fed veteran, chosen by Mr. Trump in part because he fit the president’s “central casting” image of a central banker. According to interviews with colleagues, friends and lawmakers, Mr. Powell has spent the past several months pushing the Fed toward more growth-oriented policies, not because Mr. Trump is demanding it, but because he believes economic data have given the Fed no other choice.

“One thing that has served him well, and has served the committee well, is to always take a fresh look at what’s happening,” said John C. Williams, the president of the Federal Reserve Bank of New York and vice chairman of the Federal Open Market Committee. “Not get caught up in ‘We said this in September,’ or, ‘We said this in November,’ but really to say, ‘What do we think is right?’ He’s focused on, ‘Let’s get it right, and then let’s do our best to explain it, in terms of why we’ve shifted our views.’”

Mr. Powell’s difficulties festered for much of last year, as the economy soared with stimulus from Mr. Trump’s tax cuts, more government spending and increased business confidence. Unemployment fell below 4 percent — approaching five-decade lows — wages began to climb and economic growth was heading toward 3 percent.

The Fed, convinced the economy could withstand higher borrowing costs, raised rates three times. But in the weeks leading to the Fed’s final meeting of 2018, ominous signs began to emerge, suggesting that China’s economy, and the global economy as a whole, was slowing. A key indicator based on bond prices was flashing warnings of a possible recession. Yet the Fed raised rates a fourth time at that December meeting, a move that even some Democratic economists called unnecessary.

Mr. Powell tried to pull off what Fed insiders call a “dovish hike” — raising rates while conveying to markets that the end of such increases was in sight. But in Fed forecasts released that day, it appeared that officials intended two rate increases in 2019.

Markets, already spooked, plunged during a news conference after the meeting, as investors interpreted Mr. Powell’s comments as suggesting more rate increases were coming soon, even if the economy slowed. Perhaps most worrisome to investors was his suggestion that another tool it had been deploying to remove stimulus from the economy — the shrinking of its giant portfolio of bonds — would continue on “autopilot.”