In part, the Fed has concluded the tax plan doesn’t pack a large punch. Fed officials predicted that the economy would grow at a 2.5 percent pace next year; the previous forecast was 2.1 percent.

President Trump has predicted that the tax plan could deliver 4 percent growth or more.

Apprised of those comments by a reporter, Ms. Yellen responded: “I wouldn’t want to rule anything out. It is challenging, however, to achieve growth of the levels that you mentioned.”

The Fed also has learned that it’s not so easy to increase inflation, which has remained persistently low despite a tightening labor market and strengthening economy. The Fed aims to keep prices rising by 2 percent a year; it is on pace to undershoot that goal for the sixth straight year, and Fed officials on Wednesday predicted a seventh consecutive failure in 2018.

But the Fed’s outlook is also politically convenient, even if some analysts described it as overly optimistic. Republicans argue that the tax cuts will deliver a lasting boost to economic growth by encouraging investment. They do not want the Fed to get in the way by raising rates.

Ian Shepherdson, chief economist at Pantheon Macroeconomics, described the Fed’s forecast as “hopelessly unrealistic.” He noted that the Fed was predicting that growth would be stronger than it expected, and unemployment would decline further, but without any increase in inflation.

“In short, the Fed forecasts an endless expansion, with minimal inflation pressure,” he said.

It would be relatively easy for the Fed to respond if inflation does begin to climb. A thornier problem is the concern voiced by a minority of Fed officials that the central bank is already raising rates too quickly.