Editor’s note: the following is an opinion piece by GBTV correspondent and conservative commentator S.E. Cupp.

In a 2008 letter to the editor, when he was merely an ambitious lawyer and not yet Manhattan’s District Attorney, Cyrus Vance Jr unknowingly wrote a prescient warning to his future self: “Success must be measured not simply by the number of convictions achieved by a prosecutor’s office, but also by a prosecutor’s ability to effect long-term crime reduction.”

An inauspicious year and a half into his term, and he definitely doesn’t have to worry about racking up convictions just yet. But Vance should absolutely worry about his long-term ability to reduce crime. Or win re-election.

The former criminal defense lawyer has botched nearly half a dozen high-profile cases in his short career as district attorney, embarrassing not only his office, but the city of New York and the justice system writ large.

And these humiliations should be a humbling warning to would be D.A.’s that high-minded activism and the promise of “meaningful change” are often the first casualties of ineptitude. Be good at your job — smart, steady, and fair — and don’t make promises you do not intend to keep. Or prepare for spectacular defeat.

In Vance’s case, those defeats came quickly and publicly.

A couple contractors connected to a fatal fire at the Deutsche Bank building, two police officers accused of raping a drunk woman, and two men accused of plotting to blow up a Manhattan synagogue — all the beneficiaries of Vance’s failure to temper his blood lust and zeal with smart and competent prosecuting.

And now, in what the New York Times called “another black eye” for Vance, the former International Monetary Fund director Dominique Strauss-Kahn will walk away — smiling — from charges that he raped a hotel maid who turned out to be as credible a “victim” as Casey Anthony. So eager was Vance’s office to make headlines for hammering a big fish like DSK (who probably isn’t a great guy, let’s face it) that it failed to do some basic research.

And, much as was the case in the famous Duke University girl-who-cried-rape disaster, the real consequences of prosecutorial ego visit the future and actual victims of sex crimes, who are met by skepticism and suspicion instead of sympathy.

The irony of Vance’s sorry record is that his 2009 campaign for election — backed by giddy leftwingers like Gloria Steinem, Caroline Kennedy and Betsy Gotbaum — promised the creation of a so-called Conviction Integrity Program. Now implemented, it’s been criticized as an unnecessary and expensive posture meant to make the former defense lawyer look serious, at taxpayers’ expense.

It’s hard to take Vance, who during law school campaigned for Colorado Senator Gary Hart, too seriously though. One of his few “successes” as DA is hardly anything to write home about.

Last year, Vance dedicated months of manpower and money to an undercover investigation of retail stores, including Home Depot and Eastern Mountain Sports — you know, where violent criminals love to shop — to catch them in the act of unknowingly selling illegal switchblades and gravity knives.

He strong-armed seven retail stores into avoiding charges by agreeing to remove the knives from their shelves, forfeit any profits they’d made on them over the past four years, and — get this — finance a campaign to educate the public about illegal knives. Vance said at the time he expected the operation to net the city, the state and law enforcement agencies nearly $2 million.

What good is justice for would-be rapists and terrorists when you can stick it to Home Depot?

His father, Cyrus Vance Sr, Jimmy Carter’s Secretary of State, was so mild mannered and even-tempered that he was once called “a man who leaves no footprints.” It’s a safe bet that the junior Vance will not be able to say the same, leaving New Yorkers wondering just whom he was fighting for as District Attorney of Manhattan — besides himself.