TODAY THE DEMOCRATIC Party's convention site-seers will meet here to select a city for 1984-- and if they think about it, they've come to the right place already. By selecting the best equipped and most hospitable city in the running, the members will be doing their party and its convention a favor. And, as we have said before, the opportunities for symbolism, impact and what we have come to think of in terms of party conventions as creative demagoguery are almost limitless in a city where a keynote speaker can carry on about a White House only six blocks away. On all counts, Washington should stack up best of the five cities on the committee members' ballot today. But might they choose San Francisco just to please party Chairman Charles Manatt of California?

That's been a whisper during the inspection tours of these two cities as well as Chicago, Detroit and New York. But surely there is no unit rule imposed on, or at work in, these deliberations. No serious member would discharge such an important duty in this fashion, overlooking the political and practical factors that make Washington the wisest choice for a safe, smooth, well- covered convention. Besides, as Mr. Manatt says, "I wanted Los Angeles. That's where I'm from."

For all Democrats, there is a housewarming in store here in the next year: a new Democratic Party headquarters building is scheduled to open on Capitol Hill in time for the convention. Isn't this where the party chairman should be "at home" with all the delegates next summer?

Add to that the best security any of the cities can offer, the largest number of Democratic governors and mayors in one region, the best guarantees of proper accommodations, the most attractive free activities for delegates, spouses and children and a chance to case the house where Ronald Reagan dwells--and this is where the Democrats ought to choose to do their convention business. Washington on the first ballot.