We need to knock down the legal barriers to ballot-casting

Eighteen-year-old Laura drove over to the Sudbury Town Hall recently with her father to register for the Sept. 18 Massachusetts primary. One part of the form asked for ''affiliation.'' The newly eligible voter was temporarily baffled. ''Independent?'' the clerk asked helpfully. ''Oh, no. I'm with him!'' responded Laura, pointing at her father.

If Laura picks up her option to vote in her state primary next week - and again in the US presidential election Nov. 6 - she will be running against the tide of her peers. Eighteen- to 25-year-old Americans traditionally stay away from the polls in droves - more than any other age group. In fact, the majority are not registered. They are a significant slice of the 90 million eligible voters in the United States who don't cast ballots.

Overall, only slightly more than half of those legally enfranchised citizens actually exercise their voting option. Ironically, this situation exists in a nation that stresses individual rights more than any other democracy in the world. Citizens in other free nations, including Britain, France, and West Germany, turn out at the polls in much greater proportions.

Why the low turnout in the US? Electoral pundits attribute much of it to apathy, perceptions that there are few real differences between political candidates and parties, and feelings that individual votes don't make a difference.

There are significant legal barriers, however, that keep up to 50 million potential voters from registering. And many of these need to be knocked down.

For one thing, it's high time for the US to look seriously at the possibility of setting up permanent universal voting registration - at least for federal elections. ''The (present) process is a burden on the citizen,'' insists Curtis B. Gans, who heads the Committee for the Study of the American Electorate. This private, nonprofit group conducts ongoing research on why many Americans don't vote.

Mr. Gans is a booster of permanent voting registration of citizens. He leans toward the Canadian system, which provides a government canvass, similar to the US Census, before each election to update the voting rolls. Those not included may challenge the list to attain eligibility.

But until the US adopts such automatic eligibility for all citizens, other steps must be taken to remove obstacles to registering and voting. These include:

* Simplifying the registration process. Now only one state, North Dakota, does not require its citizens to register in order to vote. Four others - Maine, Minnesota, Oregon, and Wisconsin - allow election-day sign-ups at the polls.

* Extending days and times of registration. Now some states provide opportunities to register only on weekdays and during working hours. Others maintain special registration periods on weekends and evenings in order to pick up those who can't leave their jobs or school during the day.

* Providing registration by mail. Only some 20 states do this.

* Reviewing residence requirements. Many of these could be eliminated or, at least, unified. Most states now require 30 days' residency to vote. Others vary greatly. Arizona and Tennessee, for example, mandate 50 days. New Mexico has no length-of-stay law for voter eligibility.

* Ease requirements for absentee voting. Any legitimate reason - such as travel, attending school away from home, a disability that makes it difficult to get to the polls, or religious observance on election day - should be honored. The majority of states have moved or are moving in this direction.

Of late, a number of public-interest law groups have filed suits against governors, election officials, or both to open up welfare, food-stamp, and other public offices for registration. Unfortunately, this has had partisan overtones - with principal backing from Democrats, who feel that the lower-income people who frequent these facilities are more likely to vote their way.

Further, both parties - through a variety of ethnic, youth, business, and religious-oriented groups - are conducting registration drives in an effort to sign up as many as 10 million new voters before the November election. Motivations, as might be expected, tend to be partisan.

Through all this activity, however, Americans will come to realize, one hopes , that voting in a free society is a duty and a privilege. In so doing, they will knock down the biggest and perhaps most dangerous barrier - apathy.