TALK ABOUT a thankless but wonderfully important mission--Geico has volunteered its computer experts to help clean up the most distressing, serious mess now threatening this city's hard-won democratic franchise: the Great Voter Roll Garble. The latest electoral horror involves untold missing, inaccurate or incomplete registration records at city hall. At last unreliable count, as many as 50,000 voters were in some clerical jeopardy were they to try to vote next time.

We expressed the hope, a while ago, that the best, most experienced computer team available might help unsnarl city hall's data gridlock--and Geico's is going to have a go. For no reimbursement but to defend its good name in the world of data processing, Geico--which keeps its own tapes and printouts on 1.5 million policyholders at any given time--will move in to match, merge, purge and whatever else needs doing to give the city a "clean" tape from which to proceed toward this year's elections.

Geico's team is shooting to complete its work by May 15. The rest, officials note emphatically, will be up to the city: publishing this "best possible" list or otherwise notifying every eligible voter in the District that registration must precede voting. That, of course, should be topped with saturation advertising and a telephone campaign aimed at convenient, quick, clear and orderly registration.

It is this city government followup, and not Geico's volunteer time and effort on a specific cleanup mission, that will ultimately determine how well the electoral process proceeds. But with Geico giving the city its best shot, the list of bureaucratic excuses for the inexcusable should self-destruct for good. The District really cannot afford another failure to carry out this most fundamental responsibility of local democracy.