WHO SHOULD vote? In America, the answer to that question seems obvious. Everybody should vote. Everybody.

In Massachusetts, that question needs asking and answering. This liberal commonwealth has the most restrictive voting laws of any industrial state, and it is the only one besides Illinois without mail-in voter registration. The issue has come before the legislature often since 1970 and has been shot down every time. Now, the issue will be put before the people as Question Number Six on the ballot this November.

Mail-in registration has the support of the governor, both Massachusetts senators, the mayor of Boston and a broad coalition of political groups. Yet mail-in has failed in the past due to the intransigence of officials and the indifference of the voting public. Only 2 percent of the electorate know that the issue will be on the November ballot, according to the latest Globe poll.

So who cares about mail-in? The elderly, the handicapped, people who work unusual hours, people who commute and anyone who can't afford to spend hours in a line waiting to exercise their most basic right as a citizen.

Voter registration by mail already exists in 23 states. Many of them report increased registration and turnout, and a Federal Election Commission Report in 1980 found no increase in voting fraud in any of the 23 states. It works, and it hasn't been abused.

But some people don't want more people to vote.

PARTY MACHINES, local voting officials and bureaucrats like the status quo. It is not to their advantage to have new voters messing up their neat rolls. They don't want to inject an element of surprise into election night.

Democracy works best when it really is government by, of and for the people. Mail-in is only one step in bringing the disenfranchised back into the political process, but it is a vital one. The bystander might ask, "Why do we need to register at all? Why can't we be registered automatically when we turn 18, like in other countries?"

So that we don't vote twice is one reason. Besides, U.S. citizens don't carry identification papers and don't want to.

Over a million of the eligible voters in Massachusetts are not registered. The percentage is much higher among the disabled, Blacks, Latinos, youth and the poor.

I spent four months in 1984 registering voters in low-income areas of Massachusetts cities, and found a wide variance in the outlook and cooperativeness of local officials.

In Springfield, one volunteer was deputized as a registrar, and we were able to do on-site registration at welfare and unemployment offices, as well as at housing projects. Even at the housing projects, we ran into trouble when we strayed from our designated sites.

In Massachusetts you are not allowed to register door-to-door, and yet most of the senior citizens could not make it to our tables. On Election Day, vans take them to the polling place, but they shouldn't have to take them there twice.

In other municipalities, officials were not even that generous. When we gathered the signatures on petitions for special sites to which they would send a registrar on a given day for two hours, they contrived to disqualify signatures, made excuses for long delays, and griped constantly about going to so much trouble for people who they said wouldn't understand what they were voting for.

Democracy doesn't need that much bureaucracy. Vote yes on Question Six.