Analysts quickly warned that investors were overreacting. There was little evidence in the rest of Mr. Powell’s speech that he intended to signal a change in plans.

But the market’s euphoria underscored the chairman’s struggles to strike the right pitch in an increasingly challenging economic and political environment, as President Trump attacks the Fed and the country’s growth comes under pressure. The market has been jittery over concerns that further rate increases could undermine the economy at a time when the prospects for companies and consumers may be softening.

The economy has been a picture of health, expanding at a 3.5 percent annualized pace during the third quarter. The unemployment rate has fallen to 3.7 percent, its lowest level in almost half a century. Inflation has picked up this year, and Mr. Powell on Wednesday highlighted signs of increased risk-taking in some financial markets, including lending to corporations.

But Mr. Trump has relentlessly criticized the central bank, and Mr. Powell in particular, for raising interest rates, arguing that the Fed is choking growth. Emerging signs of weakness in some parts of the economy, including auto manufacturing, agriculture and housing, are also raising concerns that the best part of the long recovery might now be in the rearview mirror.

“We’re in the 10th year of the expansion, and there are some soft points,” said Ellen Hughes-Cromwick, a former chief economist at the Ford Motor Company and the Commerce Department who is now the associate director of the University of Michigan’s Energy Institute. “The auto sales cycle has peaked, and the housing cycle also has peaked.”