Public approval of Jimmy Carter's job performance has reached its lowest mark since he became president 15 months ago, according to the nation's two major polling organizations.

The latest Gallup Poll and Harris Survey results, released yesterday, suggested that public jitters over the economy have contributed to Carter's continuing slide in the ratings.

Gallup reported that four in 10 Americans (39 percent) approved of Carter's performance, a drop of nine percentage points since the preview survey in late March.

Harris found slightly more than three in 10 Americans (33 percent) approving of the president's performance, a drop of 10 points during approximately the same time period.

Both organizations indicated that public concern over Carter's handling of economic matters played a large part in the ratings, but both stopped short of pinpointing the economy as the main cause for slippage.

While both found Carter at a new low, there were substantial differences in the actual findings.

Haris, for example, said that two-thirds of adult Americans view Carter's job performance negatively. But Gallup found that less than half said they disapproved of the president's performance.

Fifteen percent of those interviewed by Gallup declined to rate Carter. Only 3 percent of those interviewed by Harris failed to offer an opinion.

The differences may be attributed in part to the way two pollsters phrase their questions. Gallup simply asks whether respondents approve or disapprove of Carter's performance. Harris asks for a rating of Carter as excellent, pretty good, only fair or poor.

While Harris considers "only fair" to be a negative rating, a Washington Post national survey in January showed that many respondents considered "only fair" to be either a positive or a neutral rating.

The newest poll results were compiled at a time when the Carter administration is widely seen as attempting to fortify its public opinion ranking.

Buttressed by approval of the Panama Canal treaties, the president in recent weeks has moved to intensify the fight against inflation, and has talked tough on the need for tax cuts.

A separate Gallup poll within the last month identified inflation as the public's major concern, finding reflected in the new presidential job-performance poll.

The earlier poll found 55 percent - an increase of 10 percentage points in a month - believing that inflation would continue over the next year.

The latest Harris Survey found only 22 percent approving of Carter's handling of the economy. More than 70 percent disapproved; 6 percent had no opinion.

Although the new surveys put Carter at his lowest point, other recent presidents - Gerald R. Ford, Richard M. Nison and Lyndon B. Johnson - had lower ratings during their terms.