THE DISTRICT'S Sept. 12 primary election is only a little more than four weeks away, but according to the D.C. Board of Elections and Ethics - the local agency responsible for verifying and counting the votes - as many as 40,000 residents will not be able to cast their ballots. They have never taken the time to fill out a registration form. What's more, about 14,000 others haven't voted once in the past four years; they won't be able to vote in this election either until they re-register.

The deadline for District voter registration is tomorrow. For these last two days, the elections board has made the registration procedure as easy as possible. For one thing, registration forms can be picked up from any of the city's public libraries - and from some fire stations and police stations around town, too. Once the forms - post cards, really - are filled out, they can be dropped in the mailbox; they don't even need a stamp. But they must be postmarked no later than Aug. 13. For those who prefer to fill out the forms in person - or think they may need help - staff members of the elections board will be in Room 7 of the District Building (14th and E streets NW) today and tomorrow between 8:30 a.m. and 5 p.m. to give help.

The September primary promises to be fairly complicated. There are 48 candidates, five contending political parties and seven officials to be elected. What's more, the District has a "closed party primary," which means that Democrats can vote only for Democratic candidates, Republicans for Republicans and so on. Residents who want to cast their ballots as Independents will have to wait until the November election, but even they must register to do so. There's no excuse for District residents not to have their names on the voter-registration rolls - and there's still time.