DEAR Rudy Giuliani,

I'm writing this open letter to you because I'm worried you're blowing it.

I still think you stand the best chance among the candidates in the Republican field of winning the general election in 2008, and I think you have it in you to be a great president.

But something strange is going on with your candidacy - something that doesn't reflect the conduct and behavior of the Mayor Giuliani whose ferocious competence and clarity are what have led me to advocate your nomination for more than a year now. ("Nominate Rudy" is the final piece of advice I offer for all those who don't want to see Hillary Clinton become president in my 2006 book, "Can She Be Stopped?")

Many things made you a formidable mayor. They include the accomplishment known the country over - the 60 percent crime drop that turned a city in seemingly irreversible decline into something closer to an urban playground than Beirut.

Far less well-known - almost forgotten even here in New York - is that you governed New York City by knowing New York City better than anyone else in New York City.

It wasn't just the tireless energy you displayed, being present at seemingly every single fire, crime scene and trouble spot no matter the hour or day. Your command of detail was astonishing, and you could out-argue most of the politicians and pundits who came after you because you had every conceivable fact at your fingertips.

It seemed you could quote from memory every single budget line in a municipal budget that totaled $41 billion in 2001.

Every January, your State of the City Address run 60 to nd 90 minutes, and you spoke walking freely around the room with a lavalier microphone - not reading a text, but giving a talk. With all the numbers and supporting data in the right place, all unimpeachably correct.

Clearly, you developed this capacity for inhaling knowledge and retaining it from your years as a courtroom lawyer. The State of the City performances were a stunt, in a way - but a stunt that enhanced the general sense that nobody could hold a candle to you when it came to the details of governance.

You combined two essential qualities of executive leadership. You had an overarching vision of a new New York City - a safer, cleaner and more prosperous place - and you were able to marshal the institutions of city government to help effect your aim.

And I haven't even yet mentioned your magnificent leadership on and after 9/11, which also combined both the visionary and the practical in a truly stirring way.

That combination of vision and competence is what even your enemies have come to show a grudging respect for.

So where is it now?

The vision seems to be there. But not the competence.

As a presidential candidate, you seem to be winging it these days - giving off-the-cuff, ill-considered answers to delicate questions. If you keep winging it this way, you're going to fly off a cliff.

For example, the answer to your pro-choice difficulty with social conservatives on the matter of abortion isn't to blather about how much you "hate it" and then ruminate on whether the government should be responsible for helping pay for one. That's what you did last week, and you must never, ever do anything like it again - if, that is, you actually want to become president.

The answer to dealing with the abortion question is to do what you did as mayor - to master the issue the way you mastered the weird particulars of zoning law in Manhattan.

By which I mean, all the jurisprudence. All the arguments. The history of legislation on the matter. The history of court rulings. Immerse yourself in it and then argue your point from a position of strength, rather than relative ignorance.

The same is true on issue after issue. You've been speaking to adoring audiences for five years now and they hang on your every word. But, as a presidential candidate, the situation is very nearly the opposite: Your words can hang you, and many of those in the audience are hoping to serve as the executioner.

You can beat back such foes any time with the power of your intellect and your peerless ability to master and synthesize ideas - if you're willing to get cracking.

So get cracking.