IT'S TIME for a change in the way Montgomery County elects its county council. Under present law, all seven seats are filled at large, by county-wide vote. To win requires a county-wide slate representing county-wide interests. In the procssues are ignored, and a little too much influence goes to the slate- makers.

Voters will find a better alternative in Question D on the ballot in November. If it is passed, two of the council seats would continue to be filled at large but the other members would be elected separately in five districts.

The present practice of requiring all council candidates to run at large goes back to the years when the council held the executive power and exercised it through a hired manager. Under that system, in which a split council could paralyze the county government, it made sense to encourage slate-making and require everyone to face the same electorate. But that rationale vanished with the arrival of the elected county executive. Now the council is purely a legislature, and there's no more reason for Silver Spring to vote for Damascus' councilman than for New York to vote for Wyoming's congressman.

There are highly reputable county organizations that oppose Question D -- the County Chamber of Commerce, the League of Women Voters and the NAACP, to cite three prominent examples. In our judgment, they are wrong. They argue that voting by district will only encourage parochialism and regional quarrels. Really? That hasn't happened in Fairfax County, which elects by district all of its council but the chairman. It hasn't happened in Prince George's County or the city of Washington, both far more diverse than Montgomery.

With nearly 600,000 people, Montgomery is now larger than a congressional district. A single constituency of that size is much too large for a local government. Scale is crucial in the operation of democracy, and a council answerable only to the whole county can ignore the concerns of any community too small to threaten a county-wide election. The county is also too large for an independent candidate to run successfully; the financial and organizational requirements are too high. To elect five of the council members in districts of 120,000 each would represent an important improvement in democratic practice.

A footnote: the ballot also offers, as Question E, another and less satisfactory variant on the idea of district elections. It is crucial not to vote for both. If both were passed, with their conflicting plans, the whole issue would go to the courts -- which might well void both. It is Question D that deserves support.