IN THE GOOD old days, the Washington tourist season opened with Memorial Day and shut down -- firmly -- on Labor Day, and any local resident, prepared and on guard, could easily defend himself.

"Gosh, Fred, what a surprise to hear your voice and it's wonderful to know you're in Washington! But I can't talk now. The living room's on fire. . . . Perhaps next year. . . ."

Now, thanks to Washington's great new Convention Center, the threat of the sudden, unexpected call from Breezewood or Washington National will hang over this city from January to December. Now, on any day of the year, the booming voice will come on the line, "Hi folks! Guess who this is?"

I speak of conventions from experience. The Washington Board of Trade is probably proud of itself for bringing to the Shoreham last summer "The National Convention and Annual Funfest: Associated Funeral Directors of America, Inc." But did the board consider that the membership chairman of that body is my great-uncle, James Finnegan from Missoula, Montana? And did the board ever spend a weekend trying to entertain a mortician?

Uncle James, naturally, wanted to visit Mt. Vernon. Is the board aware that in 30 years of living in Washington, I have already -- as a host to visiting firemen -- spent more hours treading the vast, hot expanse of that old plantation than old George himself put in during his residence there?

I know, too, they're going to push this new Convention Center. Promote it with big staffs of P.R. types turning out news releases and brochures -- all bearing that sinister subliminal message that a visit to the nation's capital is somehow educational. For us who live here, that dubious proposition is downright dangerous.

Consider my wife's cousin Florence who decided a trip to Washington would make her children better citizens. She was standing at the base of the Monument after a long weary day dutifully checking off her sightseeing list. Suddenly she looked up in horror. "We forgot the Constitution and the Declaration of Independence!"

It was late, the rush-hour traffic was building up and I had no recourse. "Gosh, Florence, I'm sorry. They're not here. We had to send them back to Philadelphia for dry cleaning."

Always, too, my relatives arrive with both cameras and a determination to capture our historic shrines for those back home. Again, I pay. I stood for 40 minutes in front of the White House during last summer's hot spell while Uncle John, his tripod fixed and his viewfinder set, patiently waited. Not to catch the president, but hoping, rather, that some idle White House staffer might pass by "the exact spot where Sam Donaldson stands."

It was equally hot a few hours later when John planted us firmly in the middle of the steamy Mall to group together in one frame "Ma," the kids, me, the white, broad, gleaming expanse of the United States Capitol, its dome and the statue of freedom on top. Uncle John kept changing lenses but to no avail. "When I get the statue in, I cut little Janie in half." He finally reached a decision. Bisect Jane and keep freedom.

My objection to the new center is basic. Washington has too many tourists already. Stop down at the Air and Space Museum any weekend and try to get through the crowds. I, myself, have never gone back there since that terrible day three years ago when we lost poor Aunt Hattie near John Glenn's space capsule -- a tragedy that was quickly forgotten in the disaster of finding her again an hour later -- right after she got air sick while watching "To Fly."

My feelings in all this derive from my knowledge that after I have, with kindness, fed and sheltered the visiting burghers, they will go back to their towns in the heartland truly inspired and enriched by their Washington visit.

A few weeks after they have returned home, however, some local statesman will stand before them and announce, "And if I am elected, I shall go to Washington and clean up that sin-filled city with its corrupt politicians, bloated bureaucracy and lobbyist leeches" and my erstwhile house guests -- my fine relatives -- will vote for him. Every time. With great and sincere conviction.

Since I had no part in the decision to build the new Convention Center, I shall take no interest in its future. That is the business of the positive thinkers who perpetrated it.

I'm aware, though, that they, too, shall be receiving the Christmas cards with the chilling little note inside: "Tom's thinking of a couple of weeks in Florida and then a stopover on the way back for the Annual Convention of Rock Musicians to be held at your new center. Won't it be fun to see each other again !!!"

All in all I find that a rather comforting thought.