Voter turnout fell in all but two states in this year's presidential election from four years ago, and voters in 12 states cast more ballots in statewide races for senator and governor than in the race for president, a research group says.

Overall, 91.3 million Americans - half of those eligible to vote - cast ballots in the presidential race. Turnout declined in 48 states and the District of Columbia from the 1984 election, said the Committee for the Study of the American Electorate.The turnout was the lowest since 1924, when an estimated 48.9 percent of eligible Americans voted, the non-partisan research group said.

Except for a slight increase in 1984, national turnout has declined steadily since 1960, when 62.8 percent of eligible voters cast ballots, according to the committee's study. This year's turnout was down from 92.7 million, or 53.1 percent of eligible voters who cast ballots in 1984.

Continuation of the trend "will pose a serious threat to the political health of the American nation," Curtis Gans, director of the committee, said Sunday.

Turnout dropped from 1984 by more than 1 percentage point in 46 states and the District of Columbia, and by 0.9 percent in Colorado and 0.5 percent in Nebraska. It rose in two states - Nevada by 1.9 percentage points and New Hampshire by 0.7 percentage points.

Some political analysts blame the declining turnout on registration procedures that present barriers to voting, but Gans said the study found registration was not the problem.

Gans said the committee's estimate of a 50 percent turnout of eligible voters this year is based on incomplete tallies in some places, but he believes the final number will be within two-tenths of a percentage point of his estimate.

Turnout was lowest in the District of Columbia, where 36.6 percent of the eligible voters cast ballots, followed by South Carolina with 38.2 percent; Georgia, 38.3 percent; Hawaii, 42.6 percent; and Alaska, 42.8 percent.

The highest turnout was in Minnesota, where 65.3 percent voted for president, followed by Wisconsin, 61.3 percent; South Dakota, 61.1 percent; Montana, 61.0 percent; and Maine, 61.0 percent.

Turnout of registered voters also has been steadily declining, except for an increase in 1976. This year, an estimated 70.5 percent of registered voters cast ballots in the presidential election.

The study said that in 12 states, more people voted in statewide races for Senate and governor than in the presidential race. Those states were: Maine, Minnesota, Mississippi, Montana, Nebraska, Nevada, North Carolina, North Dakota, Utah, Vermont, Washington and Wyoming.

Turnout rates for other states were:

North Dakota, 60.9 percent; UTAH, 58.8; Iowa, 58.4; Vermont, 58.2; Idaho, 57.2; Connecticut, 57.1; Massachusetts, 56.7; Nebraska, 55.2; Missouri, 54.6; Ohio, 54.5; Colorado, 54.2; New Hampshire, 53.7; Michigan, 53.6; Kansas, 53.3; Oregon, 53.3; Illinois, 52.4; Indiana, 52.4; Rhode Island, 50.6; Louisiana, 50.3. Also: New Jersey, 50.1; Wyoming, 49.5; Pennsylvania, 49.4; Mississippi, 48.9; Oklahoma, 48.3; Washington, 48.1; Kentucky, 47.7; Virginia, 47.7; Delaware, 47.0; Maryland, 46.7; West Virginia, 46.3; New York, 46.0; Arkansas, 45.9; New Mexico, 45.2; Alabama, 45.1; Tennessee, 44.2; California, 44.1; Arizona, 43.9; Texas, 43.6; Nevada, 43.4; Florida, 43.4; North Carolina, 43.2.