More than 20 years ago, as a young lawyer working in the New York district attorney’s office, Cyrus Vance Jr. prosecuted the case of a young, crack-addicted black woman, who had been involved in an armed robbery.

“During the course of about a year and a half, I became convinced, because of the good work of her lawyer and the judge, that what Pam really needed was not state prison, but treatment,” he recalled Wednesday in an interview on the porch of Alley’s Store in West Tisbury.

So that is what she got.

Fast forward to the near present, when Mr. Vance announced he was running to replace the legendary, retiring 35-year veteran district attorney Robert Morgenthau. A phone call came out of the blue from that woman, offering to work on his campaign.

Turned out in the intervening years she had gone back to university, graduated with high honors, become a substance abuse counsellor, a real estate agent and “tax-paying resident in New York city.”

The lesson to take from that story, said Mr. Vance, is that finding ways to treat the underlying problems which lead people to crime is often a more effective approach than imposing ever-tougher jail sentences.

Now, in taking that view, he is running counter to the prevailing view of the American justice system over the past 30-odd years, which has resulted in the United States having the highest imprisonment rates in the world.

Let’s repeat that: in the world. Only Russia even comes close. It’s seven times as high as Canada, Australia or Britain.

Yet only a generation ago, America had incarceration rates about on a par with other developed nations. What happened to make this the most punitive nation on earth?

“Law enforcement and criminal justice go through cycles, just like economics and other things. Theories recycle, and there was a theory that what we needed was a very tough approach. Three strikes and you’re out was a perfect example,” Mr. Vance said.

(Coincidentally, New York Times columnist Nicholas Kristof Wednesday related a tragic example of where the three strikes policy has led — the case of Curtis Wilkerson, now serving a life sentence in California for his third offense of stealing a $2.50 pair of socks. California spends $49,000 a year keeping him in jail.)

“Our prison population has exploded in the past 25 years,” said Mr. Vance. “We have five per cent of the world’s population and 25 per cent of the world’s prisoners.”

But these harsh measures do not lead to effective long-term crime reduction, he said. Nationally, more than two-thirds of those people will be back in jail within three years of their releases.

The damning statistics go on. One in 100 Americans in prison. One in every nine African American men between the ages of 19 and 34. And most of them there for nonviolent offenses.

There is an alternative.

“That getting people treatment for the underlying issue that got them in trouble — whether it’s drug addiction or mental health problems, whether it’s anger management — trying to identify the factors and change that behavior, outside of a prison,” he said.

“I think that’s a very sensible way to approach law enforcement [because] if you can get them that help, then you really reduce the chances of them returning to criminal behavior.

“Then you’ve made us safer, you’ve saved us money, you’ve restored families that otherwise would have been broken up, you’ve prevented other crimes from having been committed and saved other people from being victims.”

The tough on crime cohort, of course, would decry this as soft liberalism. But Mr. Vance eschews such labeling.

“What you want in a prosecutor’s office is to promote two things,” he said. One is public safety, the other is fairness. I actually think, if you look at the questions through the lens of whether the policies will make us safer, what they see as liberal might just be smart.”

“Sending people to prison certainly is effective in terms of keeping them out of society, but the recidivism rate is very high, and prison is very costly — $60,000 or more for a year in state prison in New York State.”

Then, hardened by their association with worse people in jail, they go back to the neighborhoods they came from, “usually with no more than a bus ticket and the clothes on their back.

“Much as it is appropriate to send a lot to prison, it’s important to have a strategy to help them when they get out,” he said.

So, there you have the essence of Mr. Vance’s pitch for what he considers the nation’s preeminent district attorney’s job. More alternatives to imprisonment in the first place, more support after release.

There already has been some movement in that direction, he said. He pointed to a recent change to New York law, made on the recommendation of the state’s Sentencing Commission, on which he served.

“Just last March, the state amended its drug sentencing laws, the so-called Rockefeller laws, to permit judges to order nonviolent, drug-dependent drug sellers into treatment, as opposed to state prison. There is no more mandatory minimum sentence.”

(The Rockefeller laws, the nations’ harshest when signed by the then-governor of New York, Nelson Rockefeller, in 1973, attached the same penalties to the supply of more than two ounces of narcotics, including marijuana, as to second degree murder.)

Achieving such change, Mr. Vance said, was a slow process.

“It’s much easier to increase sentences, politically, than to decrease them,” he said. “It took years and years for the Rockefeller drug laws to be bumped down so the prosecutor does not have a veto.”

The candidate, who has worked as both a prosecution and defense lawyer over some 27 years, conceded Manhattan was one of the few places in America where he would stand a chance of election, given his views.

“I guess I would have a hard time getting elected if I were running for a prosecutor’s office in Des Moines, Iowa. My beliefs just wouldn’t track with that community.”

But in Manhattan he looks like a strong chance to win. He has sewn up most of the important endorsements, including those of The New York Times, Daily News and, importantly of Robert Morgenthau, who, at a fundraiser in Chilmark on Wednesday, called Mr. Vance “the only qualified candidate.”

The race will effectively be decided on the day of the primary, Sept. 14, among three Democratic Party contenders; there is no Republican candidate for the office.

It is the first attempt by Mr. Vance, 55, to win public office and it has been, frankly, more stressful than he expected. But, he said, he was feeling confident in the closing stages of the only office he ever wanted. He had no interest in being “a senator or governor or anything.”

“This is the one job I think I’m competent to hold,” he said, self-deprecatingly.

“It’s been in my mind a long time, but I wouldn’t have run against Bob.”

He began to prepare himself about 18 months before Mr. Morgenthau, 80, announced his intention, after 35 years, not to run again.

“There have been only three district attorneys in Manhattan in the last 75 years — Tom Dewey, Frank Hogan, Bob Morgenthau — each of them a legend,” he said.

“But I don’t view this as a 30-year position for me. I believe in limited government service. I look forward to several terms and then handing over to someone else.

“I think it is the greatest job in the world for a lawyer. The office has so much impact for good.”

If he wins, Mr. Vance will be continuing the tradition of public service of his father, also Cyrus Vance, who served the Kennedy, Johnson and Carter Administrations.

And also continuing an association with the Vineyard.

“My parents bought property on Watcha Pond in the late 1970s or early 1980s, and we have been coming here ever since,” he said.

And so it was a mix of New York legal types, seasonal visitors and Island residents who gathered on Wednesday for an unusual fundraiser — a man seeking office in New York campaigning in Massachusetts.

This visit was only for a day. But win or lose, he said, he and his family will be back in the fall.