Ordinarily, such figures would bode well for Republicans heading into the midterm elections this fall. Most Americans, however, said Mr. Trump’s policies had either hurt the economy or had little effect on it; only 38 percent said his policies had made it better.

“The overall story remains that the president is not getting credit for an economy that has been continuing on an upward trajectory,” said Jon Cohen, vice president for survey research at SurveyMonkey.

Other recent surveys show public confidence in the economy is rising, but less movement on the tax bill. A Quinnipiac University poll released last week found that two-thirds of Americans viewed the economy as excellent or good, up 3 percentage points from December. A Gallup poll published last week found that approval for the law had risen to 33 percent in January from 29 percent in December, an increase that was not statistically significant.

Supporters of the tax law dismissed its poor poll numbers in the fall, saying Americans would like it more once they began to see its benefits in their take-home pay. “The results are going to be what sells this bill,” House Speaker Paul D. Ryan, Republican of Wisconsin, told reporters in mid-December.

Many Americans won’t start to see more money in their paychecks until February, because the Internal Revenue Service issued new tax withholding guidance only last week. But a series of high-profile announcements, from companies such as Walmart and Wells Fargo, have cited the tax bill in decisions to raise wages or give employees one-time bonuses.

Many analysts, however, say the pay increases were more a result of the strong labor market than the tax law. A Morgan Stanley survey of Wall Street analysts released on Tuesday found that only 22 percent expected the companies they follow to direct at least some of their tax savings to employee compensation. By contrast, 83 percent of analysts said companies would increase share buybacks, dividends or merger activity. (Analysts could select multiple responses.)