In the history of that long and ugly war, there were few moments, if any, as poignant as this one.

As a regiment of Viet Nam Veterans Against the War poured into Washington to throw away their medals on the steps of the Capitol, John Forbes Kerry took his seat at the witness table in front of a panel of the Senate Foreign Relations Committee and delivered a speech that would move a nation. "How," he said, looking up from his prepared testimony to the senators, "do you ask a man to be the last soldier to die for a mistake?"

It was April 23, 1971. The veeterans' action that weekkend would be another one of the turning points against the crumbling consensus to prosecute the war, and overnight, John Kerry became a national hero. Many compared him to young John Kennedy, and the similarities were there in part: handsome, articulate, heir to a family that made its fortune in the China trade, a Yale graduate,a and U.S. Navy "swiftboat" commander with a pocketful of medals (three Purple Hearts, Bronze and Silver Stars. Kerry was the perfect spokesman for the militant veterans movement.

But these were the same qualities and qualifications that would pose problems for him as well.Suspect as an opportunist by the antiwar veterans rank and file, Kerry later ran unsuccessfully for Congress from Massachusetts amid taunts of "carpetbegger" from the local press. After 1972, he fell out of the limelight, and after a period of brooding, enrolled in Boston College Law School. Today he is an assistant district attorney for Middlesex County in Cambridge, Mass., working for the venerable and nearly infirm John Droney, who was appointed to the District Attorney's job on the recommendation of Sen. John Kennedy in 1959. Most local political observers in that most political of cities consider Kerry's future a bright one. The problem of image is still there, however.

"Frankly," Kerry said with some impatience during a conversation in his office a couple of weeks ago, "I am still somewhat puzzled by the attention that someone in politics attracts for taking advantage of opportunities, versus anyone else in any other industry."

Dealing with the opportunist charge by fellow veterans was "difficult," he conceded.

"I was kind of living daily in a contradiction, because to a large degree, by background, by the stereotypical labels, I wasn't supposed to be there," he said of the demonstration in 1971. "And yet by being there with those labels lent it some credibility, and I guess, brought it some focus. But at the same time, it cost me a lot, in the sense that, for all those people who said, 'That's great, he's terrific,' there was those who said, 'He's a communist, he's a traitor, or unpatriotic' . . . or whatever.

"I'tell you," he continued, "when I went to Washington to demonstrate, I really thought the worst could happen, and I turned to my brother-in-law and said, 'You know, if I ever want to run for office, this is the most disastrous thing I could be doing."

But he ran anyway.

Kerry had already taken a fling at a congressional campaign in 1970, eventually dropping out in favor of Robert Drinan, who has held the Third District seat since then. After the march on Washington, he was eager to take another run at it, and looked around for another available district.

"But the explosion of exposure in Washington really made it impossible to leave afterward," he explained, "because everybody would have said, 'Well, all he did was come down here to do this and now he's going back.' So I stayed with it the extra time to negate the sense that I was there for personal desires as opposed to helping the group."

The extra time didn't exactly hurt him. For the rest of 1971, he was courted by everyone from Dick Cavett to William Buckley and was asked, with the backing of Ted Kennedy and Michael Harrington, to become director of Massachusetts' Democratic Party State Committee. In November 1971, Drinan and Sen. Birch Bayh introduced a bill to lower the minimum age for members of the House to 22 and the Senate, 27 - which was immediately dubbed the "Kerry Amendment."

In early 1972, he moved his residence again, to Lowell, and began to chart a run for the seat being vacated by the retiring Bradford Morse.

Kerry would spend almost $300,000 in a losing bid for the seat in a three-way race. The apperances of George McGovern, Ted (and even, Caroline) Kennedy, and other luminaries were outweighed in the end by the hostility of the local press, the last-minute withdrawal of one of the candidates who threw his support to Kerry's opponent, and one extremely embarassing gaffe: Kerry's brother and a campaign aide were caught by police fooling around with the telephone lines in his campaign headquarters basement, which just happened to adjoin the headquarters of an opponent. Although the two men explained innocently that they were just checking the phones, and no official charges were brought, Kerry's opponents had a heyday with the primary-eve incident. Still, he won the primary, but lost the general election by 18,000 votes.

"Nineteen seventy-two was a downer," Kerry, now 34, said with a shrug, "but I learned some good lessons from it." Such as? "Well, you get tougher, but learn how to lose," he said, stretching back in his chair and folding his hands on his lap. He didn't seem quite at home with the notion. "Well," he said, his eyes turning steely, "I was going to add that you learn it, but you don't like it."

The mention of his name in a gathering of lawyers and staff from the D.A.'s office in the courthouse coffee shop at first elicted a rolling of eyes and embarrassed smiles, but for the most part, eventual grudging respect as they talked about Kerry's record as the septuagenarian Droney's heir apparent.

Said one 20-year veteran of the D.A.'s office, now in private practice: "He was a pleasant surprise. A lot of people expected him to fail, and he didn't. You know, it is an odd relationship, him and Droney. Kerry has brought in all those federal funds, something Droney never would've done, because of the strings attached."

(Kerry was chiefly responsible for landing a $500,000 grant from the Law Enforcement Assistance Administration for a white-collar crime program). "But Droney always said that he likes 'em young, lean, and hungry, and Kerry fits that category."

Does Kerry have a kind of star status around the courthouse? The veteran attorney laughed. "It depends on who is looking at him," he said. "The old pols don't like him, sure, but the young people like him."

"He took in the District Attorney's office," broke in one young lawyer who works with Kerry, "which had settled into a 20-year mold, and brought it into the '70s and '80s. He has brought in a team concept which allows for more flexibility and lighter case loads . . . We've attracted people into careers in prosecution rather than using the job to build their private practices."

Kerry had counted as one of his accomplishments the successful prosecution of a local organized crime figure, Howard Winter. Some people had said that Kerry trumped up the case for the publicity value.

"You have to look at it this way," said another veteran prosecutor, gesturing with a styrofoam cup. "The Justice Department's O.C. (organized crime) unit chased that guy for 4, 5 maybe 6 years, and they couldn't nail him. You can't say it wasn't good case and prosecution, because this this guy (Winter) wasn't a schlepper. It was a real good score."

"Of course, the indictments wouldn't have gone forward without Droney's say so," added another attorney knowingly "I would say that Droney was the quarterback, Kerry the halfback."

Upstairs a few minutes earlier, in the modern large office with the view of industrial Cambridge and its sprawl of gray, turn-of-the-century tenement houses, Kerry had maintained that "what I'm doing here is absolutely no different (from the veterans organizing). It's working to make the system work."

He says he's at home in the job - for now. "I don't want to stay anywhere forever. I would love to run for office," he says candidly, "although," he adds, drawing back inward, "there are only certain ones I would run for, withoug naming then . . .

"There's a point where I would say I'm ready for something else, but this is great for now."

Even the courthouse pols who didn't like him granted that Kerry would likely have a bright future in Massachusetts politics. The Commonwealth's voters seem to have a penchant for electing liberal brahmins to office.

("Miss Julia Stimson Thorne," effused The New York Times in its report on the Kerry wedding in 1969, "whose ancestors helped to shape the American Republic in its early days, and John Forbes Kerry, who wants to help steer it back from what he considers a wayward course, were married this afternoon at the 200-acre Thorne estate in Bay Shore, Long Island.") They have two children, Alexandra and Vanessa, and live quite comfortably in Newton, a well-to-do Boston suburb still represented by Robert Drinan.

"I think there are still things attractive about it," Kerry said after a discussion about Rep. Michael Harrington's wellpublicized unhappiness with Congress and his decision to withdraw from running next year. "I don't snub my nose at all at the congressional effort. I still think," he said, "there are real possibilities."

He says he resisted pressure from several quarters to run for office this year. "I didn't think that, several years ago, one of the lessons I learned in 1973 was, you know, to focus, to say, I'm going to to something, and this is what it's going to be. And I've chosen to be a lawyer, and to try to at least to be serious within the confines of this profession.

"And yes, I'd loved to run for political office. I'm not abashed about it. I would. But not any office. I don't want to just run for the sake of running. I want to run for the right thing . . .

But aren't these just the kinds of things he would have said as he ran for Congress in 1972?

"Yes," he said, with a glance at his watch. "But I was a neophyte then. . . ."

On June 28, Kerry's boss, John Droney, who walks with a shuffle, surprised a lot of courthouse pols by refusing to retire gracefully, and filed again for the elections. And he won a new term - which runs for four years. By waiting for Droney's last-minute and unexpected decision, Kerry had passed up a run at other state and federal races. So where did he see himself 10, or even five years from now?

"I really don't know where I'll be in six months."

Again last week Kerry said, "For the moment I'm very happy. I have no plan to go anywhere else. I really enjoy what I'm doing."

He thanked a reporter for calling. And then there was a pause. He laughed and added "If I get a call from Jimmy Carter I'll have you to thank."