"My God this is embarrassing," a well-connected Republican groaned yesterday afternoon. "I mean, this is horrible -- it is not good for the country."

The sudden withdrawal of Douglas H. Ginsburg's nomination to the Supreme Court because of his past marijuana smoking was one of those rare political events that sends a shudder through society. Even before Ginsburg could go to the podium in the White House press room to announce the withdrawal of his nomination, other political figures were rushing to confess their own past flirtations with marijuana.

Ginsburg's self-immolation was a profoundly embarrassing development for a president whose leadership was already under fire from many quarters in Washington, on Wall Street and from friendly governments abroad. It was an embarrassment too for Attorney General Edwin Meese III, who had pressed the choice on President Reagan, and for Ginsburg, a man now destined to go into history as the one-time dope-smoker who couldn't leap over his own past and society's taboos to a seat on the nation's highest court.

But this was more than a political matter. It was a collision among conflicting strains of the modern American culture -- in this case, the reality of widespread drug use versus our puritan, often-hypocritical moralism. Ginsburg is another victim of this year's public fixation with private lives.

He became the point man for a generation of Americans who are reaching the age when they will be asked to run society's institutions. It is the '60s generation, whose common experiences were rock music and dope and the subculture that both spawned -- a subculture dedicated to the proposition that you could never trust anyone over 30, in Abbie Hoffman's memorable formulation.

"Folks are going to have to get used to this," said Robert G. Beckel, 38, who ran Walter F. Mondale's presidential campaign in 1984. "This is the age of Aquarius growing up."

"There is a certain irony here," in the words of a prominent Republican, age 43, who is close to the Reagan White House. "The guy {Ginsburg} is too modern. He's been married twice, his wives don't take his name, he smoked a little pot, he ran a computerized dating service -- he sounds like a modern American. Are we ready for a modern American on the Supreme Court?"

What exactly happened to Ginsburg will remain a matter of debate and confusion. Many argued that his marijuana smoking was a "youthful error," in Reagan's words, that could easily be forgiven. Sen. Joseph R. Biden Jr. (D-Del.), chairman of the Senate Judiciary Committee, flirted with the idea of calling a news conference Friday to urge Ginsburg not to withdraw merely on the basis of the marijuana issue, Biden said yesterday, because he agreed with Reagan about it.

But Peter Hart, a Democratic pollster, observed that Ginsburg had admitted smoking marijuana not as a student, but as a Harvard professor -- "a professor of law" -- in his 30s. Moreover, Hart noted, the incident became a scandal in large measure because the law-and-order, antidrug-crusading Reagan administration had chosen Ginsburg to be a law-and-order Supreme Court judge.

Moreover, Ginsburg's nomination "was not on real steady legs before this {marijuana} thing," as Republican pollster Robert Teeter put it yesterday. Serious questions had been raised about his lack of legal experience, his ownership of $140,000 in cable television stock when he was handling cable television matters in the Justice Department, and the lack of knowledge about his views on constitutional issues.

"One thing you need is strong supporters," Teeter observed, but Ginsburg apparently had none in the Senate. Some of the conservative Republicans who were the strongest backers of Robert H. Bork, Reagan's first unsuccessful nominee to the court, offered the most direct criticism of Ginsburg last week.

Regardless of the merits of Ginsburg's fate, what happened to him will contribute to a sense that 1987 has been an extraordinary year for writing new rules in American public life. "This is a bizarre year," said Biden, one of its victims. "It is going to go down like '68 without the sense of tragedy, but the same sense of the unexpected."

The spectacle of a 68-year-old Brahmin like Claiborne Pell coming forth to confess a dalliance with marijuana symbolizes the new confessional phase in American political history. Lynn Cutler, an Iowan who is in Des Moines this weekend as vice chairwoman of the Democratic National Committee, observed there yesterday that dope-smoking has become "another small pothole on the way to the presidency . . . . Frankly, I think the whole thing has gone too far," she added. "Enough is enough. I've had it with this picking at things that don't matter."

Biden agreed. "Contrition is not acceptable any more," he observed, complaining that a presidential candidate today is expected to endure "open-ended {personal} questions -- did you ever in your life do such-and-such -- . . . that are contrary to our traditions of civil liberty."

Biden blamed the news media for the new fixation on personal issues. "I really think that when something like this starts to run, a sort of frenzy comes on," he said. Once anyone admits anything, he added, citing the example of Ginsburg's marijuana smoking, "it becomes fair game to ask everyone the same question." That was happening in Iowa yesterday.

"Unfortunately," Ginsburg said in his withdrawal statement yesterday, "all of the attention has been focused on our personal lives and much of that on the events of many years ago."

Personal lives do have a new place in public debate. Rep. Barney Frank (D-Mass.), reacting earlier this year to the public preoccupation with them, decided to announce that he is gay -- a disclosure that would have been unthinkable just a few years ago and that has evidently done Frank no serious political harm.

Former Colorado senator Gary Hart is out of the presidential race because of his personal life. All the candidates for president have held strategy sessions planning how they will react to personal accusations or revelations.

Hart, the pollster, argued that voters are looking for candidates with values they can embrace and that an isolated peccadillo in someone's past really isn't important. He noted a poll published in USA Today Friday in which 70 percent of the sample said the knowledge that a presidential candidate smoked marijuana in college would make no difference.

Teeter argued "six months down the road" the Ginsburg episode will be largely forgotten. There is a presidential election going on, and "there's too much real news," Teeter said.

The Republican who expressed dismay at the "horrible" humiliation of these events was less certain. "We have as weak a cast of political characters as anyone in the Western world," he said of the country's current leadership, adding: "This government still has 14 months to go, you know."