AN ADVANCE PARTY of Democrats has just given Washington a good look on its way to selecting a 1984 convention site, and though the members won't make their selection until April 21, this city seems to be much in the running as these primaries run their course. By any partisan standards of taste, frugality, comfort and safety, of course, Washington should come out on top. No region is more eager, more equipped or--given the party of local and regional officeholders around here--more politically congenial. And no place could give the Democrats a closer geographical approach to the object of their exercise, the White House.

Only blocks from 1600 Pennsylvania Avenue NW, in fact, is the big new convention center where the party's site selection and technical advisory committees convened on Friday for a 21/2-day visit aimed at pointing up the attractiveness, practicality and diversity of Washington and its environs for a smooth convention week.

By now, the advance-party members have heard all the vital statistics, including rundowns on the new facilities, ample accommodations, media availability, prime-time possibilities and top-rated, sophisticated security. And though the convention's spouse-and-offspring caucus won't be voting on the convention floor, its activities here also could be engaging; there is free access to more interesting activities in this region than in any of the other contenders.

The capital of the country and its two neighboring states are eager and ready to put on the dog for a national political convention.