BACK IN 1968, when Richaard Nixon was campaigning for the presidency, he referred constantly to Washington as the crime capitol of the United States -- his way of saying that the Democrats had been soft on crime. After all, there was only one city run lock, stock and police department by the federal government. It was Washington, D.C., and a dangerous place it was.

In a sense, the use of Washington to make a point about crime was pure Nixon. The federal government, for all its power in most areas and its ability to make war overseas, can do little when it comes to crime. Most crimes are not federal but state or local violations, and this includes most of the crimes that concern the average American -- murder, rape, burglary, robbery and assault.

Nixon persisted anyway and by now most Americans must associate the District with a lawlessness not found in other cities, except maybe New York. It would be wonderful to say that's not true -- that murder, rape and general mayhem are unknown here and that the city has gotten a bad rap. But the facts are otherwise -- not as stark as Nixon would have had you believe, but nothing to boast about. Washington, like many big cities, can be a dangerous place.

BUT IT IS ALSO the place where some of the programs now being advocated by the Reagan administration to fight crime already exist. The two principal administration spokesmen when it comes to fighting crime, presidential adviser Edwin Meese III and Attorney General William French Smith, have both come out for preventive detention and for putting more criminals behind bars and for keeping them there a longer time.

The administration has other ideas when it comes to fighting crime, including a modification of the exclusionary rule under which judges throw out illegally seized evidence. But the bulk of the package outlined by Meese and Smith is nothing new. It already exists in, of all places, Washington, D.C.

Washington is the only place in the country that already has preventive detention. In Washington, defendants can be detained prior to trial if there is a good chance they would commit another crime while out on bail or wouldn't show for trial at all. The law, though, is infrequently used. In the first place, proving that likelihood of someone committing a crime is one of the great futile exercises of our times and, second, invoking the law means that the defendant has to be brought to trial within 60 days. Often a prosecutor would rather have the defendant out on the streets than rush him to trial.

But Washington has more than preventive detention. It also has jails galore. It has, in fact, the highest per capita incarceration rate of any city in the country -- something like 1 percent. What that means is that at any given time, Washington has more of its citizens in the clink that any other city in the country.

WHEN YOU COMBINE preventive detention with a high incarceration rate and with a large police force, you would think that the crime rate in Washington would be low, maybe near nonexistent. Lambs would lie down with lions and women would walk the streets at night and little old ladies would not tremble when they did their grocery shopping.

But, lo and behold, Washington is not crime-free. In fact, in 1979 the FBI ranked it sixth in its rate of violent crime, behind Baltimore, New York, San Francisco, Detroit and Cleveland -- not quite the crime capital of the country, but not quite Grovers Corners, either. It is a typical big city and, while you could argue that without these measures, without preventive detention and lots of people in the calaboose and oodles of cops, the crime rate would be even higher, it is nevertheless clear that it takes more than what Meese has suggested to turn a city into a safe place.

But you don't get any inkling of that from the administration. From both Meese and Smith comes nothing but an updated version of Nixonian crime fighting: You gotta get tough. Well, maybe you do, but you also have to get imaginative and you also have to fight crime where it starts -- in poverty and despair and ignorance. Otherwise, you will never have enough cops or enough jails, but always plenty of criminals. We Washingtonians know that. We have seen the future. And it doesn't work.