Gov. Mario M. Cuomo (D) of New York came to Washington over the weekend for a social occasion, the wedding of his son Andrew to Robert F. Kennedy's daughter Kerry. But since there is nothing purely social in Washington, the wedding was also a political event, at which the future happiness of the young couple was almost as much discussed as the political future of the bridegroom's father.

The wedding at St. Matthew's Cathedral -- which, it was well remembered, was also the scene of Jack Kennedy's funeral -- was a spectacular affair, and the advent of Kerry Kennedy to the Cuomo tribe was seen as a clinching argument for her new father-in-law's candidacy.

Surely she is a young woman of originality and flair, and not afraid of breaking precedent. After a parade of little boys and girls from both tribes, all in white, bearing rings, strewing rose petals, and a file of 15 bridesmaids and 15 ushers, a blare of trumpets announced the bride. There stood Kerry Kennedy, a brown-eyed blonde, like an apparition. The sun streaming in from a window high above the organ shone through her cloud of veil and glinted on the brilliants on her hoop-skirted satin dress. She was alone, and as she made her way up the long aisle, the congregation burst into applause. The Cuomos were getting a taste of Kennedy flair.

At the reception at Hickory Hill, the governor and Sen. Edward M. Kennedy (D-Mass.), who is, of course, his clan's perennial first choice for president, sat in long intent converse. Cuomo, if he runs, can depend on the Kennedys' famous tribal loyalty. He was for Jimmy Carter in 1980, when Teddy made his move, but now he is, so to speak, one of them, and they would like him to go for it.

Someone caught up in the rich mix of ethnic rivalries and cross-currents asked a Cuomo in-law what it was like to marry into that tribe.

"Great," was the dry reply, "if you don't have any strong opinions."

The Kennedys have a history of overwhelming their in-laws, but there was a general feeling that, in the Cuomos, they had met their match. Matilda Cuomo, the govenor's wife, danced up a storm with her son Christopher. Ethel Kennedy and her son Joe, the Massachusetts congressman, took to the floor and boogied with such vigor it seemed they would take off. Rory Kennedy, the bride's youngest sister, offered a warm, funny toast. Christopher Cuomo, the bridegroom's younger brother, did the same. This, obviously, is a merger, not a takeover.

Up and down the green lawns of Hickory Hill, where dogs with white satin bows trotted around and flower girls rode on giant swings under the oak trees, the talk was of politics, and all was consensus: Mario Cuomo's time has come.

"The more you tell him that Bush is invincible, the more he wants to take him on," said a member of the Cuomo entourage.

One thing is certain: Cuomo thinks he has located the winning issue for the Democrats. He found out himself when he spoke before the Democratic State Convention that renominated him for governor on June 5 in Albany. He brought the house down when he spoke two lines about the S&L bailout. The deregulated bankers "stole everything in sight," he said, and there was "an explosion."

Cuomo explained, at a brunch he and Mrs. Cuomo gave for their out-of-town guests the day after the wedding, that "the S&L bailout is a heavyweight champion issue for Democrats, not in 1990 but in 1992. It has to be talked and written about before you bring it to the fore. It's in training now."

Democrats in Washington think it's easy for him to say -- he brings clean hands to the issue. They are too conscious of Democratic involvement: the four Democrats who are part of the Keating Five, friends of S&L kingpin Charles Keating; former House Banking Committee chairman Fernand St Germain (D-R.I.), who helped raise government guarantees on loans from $40,000 to $100,000; former House Speaker Jim Wright, who intervened for S&L pals in Texas.

Cuomo thinks the issue will appeal to the country's sense of fairness. "Some poor guy steals a pair of shoes and is put away for a couple of years. We have put a record number of young minority males in jail. And what are we doing about these bums, who robbed us blind?"

Cuomo is going to write a handbook about the S&L issue, one that he hopes will teach Democrats how to handle it on the campaign trail.

By 1992, the public will be ready to make its big decision on the merits of the case. So presumably will Mario Cuomo, a man who acquired a host of glamorous new allies last Saturday.