The only fireworks at the White House were nonpolitical last night as President and Mrs. Reagan hosted their annual picnic for members of Congress. It was reportedly the first time in history that peaceful pyrotechnics have lighted up the sky directly above the Executive Mansion. British ordnance in 1814, of course, was another matter.

"I hope they wet down the roof," Reagan quipped afterward of the bombs bursting in air. The pyrotechnical Zambelli Brothers fired the rockets from just inside the fence off Pennsylvania Avenue while the president and his guests watched from picnic tables on the South Lawn.

The dazzling show, with musical accompaniment by the Marine Band, climaxed an evening of politicking and backslapping, what Reagan described as a bipartisan gathering that helped "build a spirit of unity and amiability."

"Only in America can a president of one party get together with congressional members of both parties for a friendly feast right in the middle of a heated political struggle," he said, a reference to the confirmation hearings for Supreme Court nominee Robert Bork.

Things were so friendly the Republicans were even speaking kindly of the departed. Like Sen. Joe Biden, who ended his presidential candidacy yesterday.

"I was disappointed. I thought maybe Joe would turn that to his advantage somehow," said Sen. David F. Durenburger (R-Minn.). "I don't condone plagiarism or some of the other things he was accused of, but we're going to knock off all our presidential campaigns. Adultery was bad enough, but now we're moving down to plagiarism and exaggeration. I mean, what's next?"

White House Chief of Staff Howard Baker said, "My heart goes out to him {Biden}. I count him a friend and I regret that it came to this." Asked if he'd ever borrowed a phrase from his oratorical father-in-law, the late Illinois senator Everett Dirksen, Baker said, "Absolutely! Shamelessly!" And did he credit Dirksen? "Well ... most times."

Sen. Robert Dole (R-Kan.), who intends to announce his own presidential plans Nov. 9, remarked wryly that plagiarism may not always be unwelcome. "A lot of people wish I would do that instead of using my own quotes," he said.

President Reagan, asked if he ever borrowed oratory from anyone else, said, "Yes, but I think I usually put it in quotation marks and give credit. It's safer that way."

Last night he was especially safe.

"I can't help remembering Lloyd George's observation," Reagan said, "that trying to enter an alliance with one of his political opponents was like going for a walk with a grasshopper. I know that some of my best friends from time to time think that I've been led both down and off the garden path by, if not a grasshopper, perhaps the occasional gypsy moth or even a boll weevil."

Rep. Robert H. Michel (R-Ill.) said yesterday he was making some comments on the debt ceiling and "I used a Churchillian quote to make no mistake about whom I'm quoting in this critical time in history. It's getting to the point where if you use any line of phraseology that seems to be familiar, you'd better be attributing it."

There was also the matter of Biden's academic record, and just what it really was. Reagan, asked if he had ever flunked anything, said, "No, I don't recall. {But} I barely got by, as you know, I've said many times. Two years ago I was given an honorary degree by my alma mater and I told them I thought the first one was honorary." Asked if he felt sorry for Biden, the president said, "You wonder who's next."

Others wondering included House Minority Leader Michel, who said, "Hart here, Biden there. They may be left with Jesse Jackson."

Durenburger suggested darkly, "There is some kind of plot around in the country. The Republicans are assuming that one of their candidates is about to get it next." Asked who might have furnished to the press the controversial TV tape comparing Biden's speech quotes to those of British Labor leader Neil Kinnock, Durenburger said he didn't know. "But some old-timer told me today he was convinced that Richard Nixon was alive and well and working some place."

And on the subject of being alive and well, what about Soviet Leader Mikhail Gorbachev, who has not been seen in public for seven weeks?

National security adviser Frank Carlucci claimed ignorance. Nancy Reagan said the American press "wouldn't let us out of sight for six weeks," and said she may ask son Ron to find out something when he goes to the Soviet Union next month with ABC-TV's "Good Morning America."

Asked if it was really Gorbachev's signature on the recent letter he sent to President Reagan, Mrs. Reagan laughed and said, "I never thought about that." Carlucci, however, said, "It looked genuine to me."

White House press spokesman Marlin Fitzwater said the State Department says Gorbachev is in fact, alive, if not well.

"We're having a summit, whether he comes or not," Fitzwater said.