In Washington, when the probable next chairman of the House Rules Committee holds a $500-a-ticket pool party on a cold, windy autumn night, a lobbyist doesn't have much of a choice.

He either goes or begins to think about divine intervention getting his bill through the Rules Committee in 1982. There might be some other ways, but last night about 200 of this town's power lobbyists took no chance.

Rep. Gillis Long (D-La.) raised $100,000 from just about every special interest around. There were the home builders, the retailers, the utilities and the commodities.

"Gillis sure does know how to raise money," drawled Rep. John Breaux (D-La). "One day I was sitting at his desk and started playing with all his telephone buttons. Why, heck, I raised $50,000 by accident."

That was a joke. And so is $50,000 in Louisiana politics. Last year's gubernatorial race soaked up $25 million. And there are only 4 million people in the whole state.

"This could be my toughest one yet," said Long, who already has $300,000 tucked away for next year's election. "I just have to scare these young fellows away."

The fund-raiser was held poolside at Watergate East, where the brisk air blew the skirts and chilled the stuffed mushrooms.

"I'm absolutely freeeezing, darlin'," oozed one tiny woman, pulling her little mink stole around her. Her dress was flying a la Marilyn Monroe in "The Seven Year Itch," but not one shoulder-length hair moved.

Only a handful of congressmen showed up, and the consensus was that everyone stayed home to watch President Reagan's economic address. Even the Democrats. But the Speaker came. He got most of the attention. He always does.

"Excuse me, Mr. Speaker," said a woman pushing herself in front of Tip O'Neill (D-Mass.). "I'm from Mary Rose Oakar's D-Ohio staff. We have your picture hanging in our office."

"Oh, she's a beautiful lady," said O'Neill, grabbing the woman's hand. "If the world had more people like Rose Mary, it would be a beautiful world."

"Mary Rose."

"What did I say? I always call her Mary Rose," said O'Neill.

"Oh, she really doesn't care," said the woman, "just as long as you talk to her."

By 8:30 the place had cleared out and even O'Neill was heading across the street to watch the speech. "The honeymoon is over," he said. "They're in real trouble."

Rep. Bill Alexander (D-Ark.) put it this way: "David Stockman just doesn't know what he's talking about. He's like a bad football coach. He knows all the plays, but he just can't score any points. The economic plan isn't going to work." He swallowed a stuffed mushroom and walked away.