Despite generally broad support for his economic proposals, President Reagan receives only a modest vote of confidence from his fellow Americans, according to a new Gallup poll.

In a nationwide Gallup personal interview survey completed Sunday, 59 percent of the public expressed approval of Reagan's early record, while 24 percent disapproved and 17 percent were undecided.

This approval rating is lower than those found by Gallup for other elected presidents two months after they took office, and the disapproval rating is substantially higher.

Greater proportions of Americans approved of Jimmy Carter (75 percent), Richard Nixon (65 percent), John Kennedy (73 percent) and Dwight Eisenhower (67 percent) two months after their inauguration than is the case with Reagan.

None of those presidents had a disapproval rating of more than 9 percent, compared to Reagan's 24 percent.

The poll, in which 1,363 people were interviewed nationwide, showed the proportion of the public that disapproves of Reagan's job performance growing faster than the proportion expressing approval.

In the Gallup organization's first assessment of Reagan's popularity, conducted 10 days after his inauguration in January, 51 percent of those interviewed said they approved of his performance in office, 13 percent said they disapproved and 36 percent withheld judgment.

Thus, Reagan's popularity has risen by 8 percentage points, but the proportion who disapprove of his actions has grown by 11 points with a decline of 19 points in the undecided vote.

Voter groups that heavily supported Reagan in last November's election understandably tend to hold him in higher approval now.

For example, the Gallup Poll's post-election analysis showed that proportionately more men than women voted for Reagan in November, and his current approval rating among men is significantly higher than among women.

Major disappointment with Reagan is expressed by nonwhites. Among them, 24 percent say they approve of his job performance, 45 percent disapprove and 31 percent express no opinion.

Among whites, Reagan has a 64 percent approval rating, 21 percent disapproval and 15 percent undecided.

Interviewing in the poll was conducted last weekend in scientifically selected localities across the nation. For results on a sample of this size, one can say with 95 percent confidence that the error attributable to sampling and other random effects could be three percentage points in either direction.