Every year about this time the Eastern Montgomery-Kensington-Wheaton Democratic club holds a beer bust at the country farmhouse of Blair Lee III. And, every year, Maryland Gov. Marvin Mandel is billed as the guest of honor, shows up, and gets rained on.

This year's-party appears to be in keeping with that tradition. It will be held at Acting Gov. Lee's place in northern Silver Spring next Saturday night. Thousands of brochures have been distributed trumpeting Mandel as the guest of honor. Everyone expects it to rain.

But there is one slight hitch.

Between now and Saturday night a jury in Baltimore could find Mandel guilty of political corruption charges. This possibility embarrasses the Montgomery Democrats. So why did they make Mandel the guest of honor?

That question was put to a worker at the Montgomery County Democratic Central Committee headquarters in Kensington yesterday. She said nothing for 10 or 15 seconds. Then, hestitantly: "I wouldn't touch that one with a ten foot pole."

For Helen Peacock, one of the movers and shakers of the E.M.K.W. Democratic Club, the question is awkward but answerable. "Well, look at it this way." she said. "We've had these beer busts for four or five years and Marvin's come to every one of them.

"And it rains and rains and he stands out there in the rain without complaining."

"Last year he had to go stand under the shed for a while. He turned to me as though I was responsible for the rain and joked, 'Why don't you go out west where they can use you?'"

"So we figured that if he was good enough to come stand in the rain all these years he was good enough to have as the guest of honor again."

Peacock said the decision to invite Mandel as the guest of honor was made last May, when it was impossible to foresee that it could amount to a post-verdict wake or celebration. "We look at it as a matter of courtesy," she explained.

"A lot of us older folks in the county think inviting Mandel was the only courteous thing to do. We couldn't prejudge him, could we? And you've got to have elected officials. Who would come if it were just a party in my backyard?"

Despite this courtesy, however, it is unlikely that Mandel will actually appear at the beer bust. For one thing, he has yet to respond to his R.S.V.P. invitation. For another, Saturday happens to mark the third anniversary of his marriage to his second wife, Jeanne Blackistone Mandel.