Although President Carter's popularity is at a low point, he edges out his two major Republican adversaries in early test election runs.

Carter leads Ronald Reagan 50 to 46 percent, with a 4 percent undecided, and he leads Gerald R. Ford 51 to 45 percent, with 4 percent undecided.

An earlier Gallup report showed Reagan trailing Ford 40 to 30 percent in the nomination choices of Republican voters. Reagan does as well as Ford in test races against Carter because of Reagan's strong showing in the South.

In the South, Reagan leads Carter 52 to 44 percent, with 4 percent undecided. Ford trails Carter by 50 to 45 percent, with 5 percent undecided.

Carter leads Reagan and Ford outside the South, and by about the same percentage.

It is important to bear in mind that these test races are indicative of present strength only of potential candidates.

The current findings are recorded at a time when Carter's popularity rating is at his low point to date, with 39 percent expressing approval of his performance in office.

Here are the questions asked in the survey:

Suppose the presidential election were being held today. If President Carter were the Democratic candidate and Gerald/Ronald Reagan were the Republican candidate, which would you like to see win?

Those who named another person or who were undecided were asked:

As of today, do you learn more to Carter, the Democrat, or to Ford/Reagan, the Republican?

Following are the results, based on registered voters and assuming the 1976 tournout ratio: CARTER VERSUS REAGAN

Carter 51%

Reagan 46

Other 1

Undecided 3

On a two-way basis (with the undecided vote allocated equally to the two men), the results are: CARTER VERSUS FORD

Carter 52%

Ford 45

Other 1

Undecided 3

On a two-way basis, the results are:

Carter 53%

Ford 47

By way of comparison, the popular vote for the major party candidates in the 1976 election was:

Carter 51.1%

Ford 48.9

The results reported today are based on personal interviews with 1,116 registered voters out of a total sample of 1,574 adults, 18 and older. Interviewing was completed in more than 300 scientifically selected localities during the period March 31 April 3.