President Clinton gives the '94 Brady Law much credit for lowering the crime rate. In fact, the White House has never offered clear evidence that the law has done anything of the sort.

Even the frequent claim that background checks have prevented criminals from getting guns is crumbling under evidence the Clinton administration may have falsified data.

This isn't Clinton's ''editing mistake,'' in which he said that the Brady Law has ''stopped hundreds of thousands of felons, fugitives and stalkers from buying handguns every year.'' Clinton's own Justice Department puts the number at 69,000 in '97.

But that figure may be wrong, too. The Justice Department may have misreported the numbers for political ends.

Take the fall in the crime rate. Remember, the murder rate began to drop in '91 - before Clinton took office and years before the Brady Law took effect.

But that's just the least of it. In recent weeks, The Indianapolis Star reported that Justice overstated the number of handgun sales blocked in Indiana by more than 1,300%. The numbers in Arizona were high, too, but by only 30%.

The Justice Department refers to selected state police agencies as its source. But some of these very same agencies denied that representatives ever spoke with Justice. ''If they're saying we had that many stops,'' said Bruce Bryant of the Indiana State Police Firearms Division, ''there's no way in the world it could be that high.''

The White House has had its Brady numbers challenged in the past. Previous estimates of the number of criminals stopped from buying a gun were skewed. The General Accounting Office brought these errors to light in '96. It showed that initial rejections numbered 60,000 - not 100,000, as the White House claimed. Most of those rejections were due to technical glitches or paperwork errors, and were eventually corrected. Some 3,000 rejections were due to convictions for violent crimes.

These 3,000 rejections could be important, even if the figure isn't even close to the administration's estimates. But that number still isn't good evidence that the Brady Law has reduced crime. In fact, the vast majority of criminals get their guns from the black market. That has been true before and after the law passed.

Even police are skeptical that the law has reduced crime. A poll from the National Association of Chiefs of Police in May found that 85% of police chiefs believe the law hasn't cut down on gun crimes.

In my book, I provided the first systematic evidence of the Brady Law's impact during its first year. I looked at crime rate data for all 3,054 counties in the United States from 1977 through 1994. I accounted for arrest and conviction rates, prison sentences, changes in gun laws, income levels and poverty rates, unemployment and illegal drug prices. The result? The Brady Law had no significant impact on murder or robbery rates in its first year, but was associated with a slight increase in rapes and aggravated assaults.

Fears that waiting periods will interfere in particular with women's ability to get a gun quickly to defend themselves appear justified. My study of state waiting periods and background checks suggested that the longest waiting periods - such as those in California and New York -are the most dangerous.

But despite the evidence against the Brady Law, Clinton is using the law's dubious success to push an extension of the five-day waiting period beyond the law's scheduled November phase- out. And the president is using the law to tout even more gun-control measures - new taxes on gun sales to pay for background checks, as well as rules designed ostensibly to keep guns out of minors' hands. President Clinton may sincerely believe that such measures promote public safety. But the ends don't justify the means: If Justice Department data are being reported falsely, it poses a real threat to any honest evaluation of U.S. crime policy.

Mistakes can happen. But so far, the Justice Department's errors have grossly exaggerated the Brady Law's success.

John R. Lott Jr. is the John M. Olin Law and Economics Fellow at the University of Chicago School of Law and author of ''More Guns, Less Crime'' (University of Chicago Press, 1998).