IN ONE RESPECT anyway the Democrats seem bent on revising their rules in a way that can only be described as playing to their weakness, not their strength. As David Broder pointed out on the opposite page recently, their strength as a party is at the local level. Voters in four of the past five elections have refused to entrust the Democrats with control of the White House and the executive branch. But voters all over the country show no reluctance to give Democrats the everyday responsibilities of local, state and congressional government.

The same voters who reelected Ronald Reagan elected Democrats in 253 of 435 congressional districts. The Rocky Mountain states are solidly Republican in national elections, yet all but one have Democratic governors. Democrats are the majority in 66 of the 99 state legislative bodies. Thousands of Democrats are elected to local office. If the GOP's strength lies in its national stars, the Democrats' is in their state and local benches.

Yet the Democrats' fairness commission, set up as part of a preconvention compromise by Mondale, Hart, and Jackson operatives, has as its mission the reconsideration of the last commission's rule setting aside some 500 national convention seats for state and local party and elected officials. If this isn't typical of the party's bent for self-destruction, you have to ask yourself what is. It's as though the Democrats had determined to ensure that those among them who know how to win elections and govern don't have places at the national convention.

Of course, simply seating local officials doesn't solve the Democrats' problems. They would also need to ponder just how these men and women got elected and how they govern -- and how they respond to the demands of the caucus groups that inhabit the Democrats' national politics.

It is in the interest of a party to arrange its nominating process to strengthen the power of constituencies capable of delivering votes in the general election. Trust the national Democrats to have a process that gives too much power to caucus groups incapable of delivering and too little to elected officials who have proved themselves capable of winning elections and of governing in the long term.