Boston — Nearly one-third of all adult Americans were ineligible to vote for president or any other office in 1980 because they were not registered. And close to 12 percent of the 105 million who qualified sat out the election.
This situation, although a modest improvement over 1976, is of increasing concern to political leaders and civic groups across the United States.
With the 1984 presidential election but 20 months away and various state primaries even closer, ways of encouraging more citizens to register and vote appear to be gaining the attention of lawmakers.
Proposals to bring the ballot closer to many of the 52 million or so men and women who have not signed up to vote are pending in at least a dozen states.
Particularly plentiful are measures to make both registration and absentee voting less complex. A number of states, including Florida, Illinois, and Massachusetts, are considering legislation providing so-called post-card voter registration. Variations of this are already in effect in 21 states and the District of Columbia.
Critics contend post-card registration carries a greater possibility of fraud than the system of requiring voters to sign up in person. Some are concerned that it would lead to absentee voting abuses.
Supporters of mail registration, however, insist that any increased risks are overshadowed by the benefits to those who would otherwise be unable to sign up because of illness, working hours, or distance from registration locations.
Proposals in several states include efforts to permit citizens to both register and vote on the same day. So far, this instant-voter arrangement is restricted to North Dakota (where there is no registration procedure at all), and Maine, Minnesota, Oregon, Wisconsin, and Wyoming, where both formal qualification and ballot casting are allowed on the same day, sometimes in the same place.
While neither endorsing nor condemning this setup, leaders of the national League of Women Voters and others promoting maximum citizen involvement in elections view it as something that merits consideration, ''providing safeguards . . . are built in.''
Virginia Schwartz, a national board member of the league and its voter service chairman, suggests that strict identification requirements could safeguard the integrity of the ballot box under either post-card registration or same-day registration. Also needed, she says, is provision for ''challenged votes to be set aside'' and not counted until the question of their legitimacy is resolved.
She notes that states are expanding their registration outreach in various ways. California, Michigan, and Ohio, for example, have in recent years made it possible for people to sign up to vote when they apply for a driver's license.
A similar arrangement, approved through a voter-initiative petition on last November's ballot, is soon to begin in Arizona. The measure's future, however, is uncertain. Legislative foes are pushing a 1984 ballot proposal that would wipe it off the books. Much of the opposition stems from a provision in the new statute which forbids the removal of voters from the registration rolls when they fail to vote in a general election as long as they retain a valid driver's license.
The move to put the initiative repeal proposal on next year's ballot is spearheaded by Republican lawmakers. The ''motor-voter'' initiative should not become a partisan issue, holds John Anderson, executive director of Arizona Common Cause, whose organization supported its passage and is determined to ''stick by it,'' pointing out that his state has one of the lowest voter registration percentages.
Arizona has been among several states, many of them in the West, which require qualified voters to register after missing an election.
While considerable progress has been made in recent years toward removing or at least lowering barriers to absentee voting, 13 states continue to require that such ballots be notarized.
This, according to Henry Valentino, director of the Federal Voting Assistance Program, poses particular problems for military personnel, their dependents, and other US citizens based overseas. Many are located in areas where there are no notaries or where such services are so costly it tends to discourage qualified Americans from voting.
''Our responsibility is to help remove such barriers so that everyone who is eligible can use an absentee ballot, and as conveniently as possible,'' explains Mr. Valentino. He points out that more than 5 million Americans, including members of the armed forces and their families, are living outside the US.
He notes, too, that 26 states plus the District of Columbia accept absentee ballots without notarization. Another 11 permit another citizen to witness the authenticity of such votes.
Moves to drop or modify this requirement are on legislative dockets in Massachusetts, Tennessee, and several other states. Also being pushed are measures to provide more time for those overseas to file their absentee ballots.
Because of the slowness of mail from some parts of the world and the time involved in applying for and sending off an absentee ballot, Mr. Valentino favors having all states extend the time frame to 45 days, instead of the usual 30 days.
Noting that 49 percent of eligible military personnel overseas cast absentee ballots in the 1980 presidential election, compared with but 41 percent in 1976, he holds that the figure ''should be a lot higher.''
States weighing changes in voter registration or absentee voting requirements include Arkansas, Connecticut, Florida, Massachusetts, New Jersey, New York, Oregon, Pennsylvania, Rhode Island, South Carolina, Tennessee, Texas, and Virginia.