If Americans could vote on major issues facing the nation, as well as candidates, voter turnout in national elections -- now the worst of any major democaracy in the world -- could improve substantially, a recent Gallup Poll indicates.

This conclusion is based on the results of a polll in which national samples of those who voted in the 1980 presidential election and those who did not were questioned about the related proposals that have been offered as ways of increasing turnout in national elections.

When non-voters were asked if they would be more likely or less likely to vote in national elections if they could vote on important issues as well as candidates, as many as half (48 percent) said they would be more likely to do so.

The implications of these findings are far-reaching: If half the non-voters who say they would be more likely to vote actually did so turnout would increase to almost 80 percent, a level equal to that found in other major democratic nations.

Turnout in the 1980 presidential election was the lowest in 32 years. In fact, only 54 percent of those eligible took the trouble to vote in November.

As the survey results indicate measures that would provide the public with avenues for direct expression of their views might well enhance their participation in candidate elections. This may have particular application in election years similar to 1980, when many prospective voters were less than enthusiastic about the major party nominees.

Further evidence of the appeal of voting on issues as well as for candidates is seen in the results of a question on a constitutional amendment, described by its sponsors as the Voter Initiative Amendment. According to this amendment, a national referendum would be held on any issue when 3 percent of all voters in the previous presidential election signed petitions demanding such a vote.

This amendment is favored 2 to 1 among all persons interviewed, with voters and non-voters in last fall's election holding similar views.

A recent report by the Committee for the Study of the American Electorate indicated that most conventional proposals for increasing voter turnout have had little effect. Participation was not notable higher in states that used postcard registration or allowed voters to register at the polls on Election Day.

The current survey results lend support to the view of proponents of the initiative and referendum that offering citizens an opportunity to vote on major issues of the day would encourage greater participation.

The latest results are based on in-person interviews with 1,553 adults, 18 and over, conducted in more than 300 scientifically selected localities across the nation during the period April 10-13. For results based 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 3 percentage points in either direction.