In a crystal tenor, sure and true, former Democratic congressman Jimmy Symington opened his act in the breezy courtyard of Decatur House last night, while Mike Deaver and Charles Wick played a duet on the piano. He crooned sweetly, "Let it rain, let it pour, it ain't gonna rain where I'm heading for . . . "

But he was mistaken. Very large drops, in fact, sent 300 primly attired politicians and socialites--gathered to raise money for Second Genesis Inc.--scattering for shelter under fortuitously set up tents. This did not lessen the fun, however. It merely crowded the buffet tables.

"I want to thank all you wonderful people who are not on food stamps who have contributed tonight to the march on drug abuse," International Communication Agency Director Wick, emcee for the evening, told the crowd.

At $75 a ticket, a melting pot of social, business and administration officials helped raise more than $35,000 for Second Genesis, a nonprofit drug rehabilitation treatment center. The high turnout of administration officials, no doubt, reflected Nancy Reagan's keen interest in fighting drug abuse.

Symington and his wife Sylvia took the microphone first and were joined soon after by White House Deputy Chief of Staff Deaver, who, in the words of one guest, "plays a mean piano." Wick filled in with Deaver on the chords, and everyone swooned for about a half hour.

"Now this is really good piano," said Mary Jo Campbell, a Republican. "We sure do have a lot of talent in the Republican Party."

"In piano playing, there's no comparison," said Robert McNamara, a Democrat.

From "Shine on Harvest Moon" to "Has Anybody Seen My Girl?" the musicians had some trouble getting the crowd in the tent to sing along. It wasn't until Deaver played "God Bless America" that everyone finally stopped talking and joined in the singing. Quite loudly.

Symington said he has been singing since grade school, although his voice changed when he was 14 "and that caused some confusion." Another guest, Peter Gilsey, attended St. Bernard's private school in New York with Symington, so the two men sang the school "Baseball Song" for a reporter.

Symington described his friend as a "local civic leader and financial adviser." In fact, just moments before, Gilsey had informed White House Chief of Staff James Baker that the "prime market had bottomed out." Gilsey's going into bonds next week--$2 million worth.

It eventually stopped raining, but the preponderance of expensive silks and precision haircuts made venturing out high risk. So the smoked salmon and piles of pink roast beef were rather popular.

"I am so proud of you," Dr. Jim Elder gushed to his old friend, the new chief of protocol, Selwa (Lucky) Roosevelt. "I hope you've starting keeping a journal."

"You know, I should. I haven't written down one word yet," said Roosevelt.

"Do you have the rhythm of the job yet?" asked Elder, concerned that Roosevelt might be getting too tired.

"Almost. But not in my legs," said Roosevelt. "In fact, I think I'll go sit down over there with Jane Weinberger."

Before the reception, many of the guests attended a White House lawn concert across the street, where the Manhattan/Downeast Chamber Orchestra played. "I sat next to Schuyler Chapin from the Met and I was so excited I couldn't see straight," said Scooter Miller.

"I think I should have gone," lamented Weinberger, whose husband, the defense secretary, was due back from Saudi Arabia last night. "I hope my absence wasn't noticed."

The party was originally scheduled for the Saudi Arabian Embassy under the patronage of Nuha Alhegelan, but was moved at the last minute due to the fighting in the Mideast. "Mrs. Alhegelan is part Syrian and part Lebanese," said the embassy's social secretary, Helen Smith. "She thought it inappropriate to have the party there because of the tragic loss of life at home."