From the moment the U.S. Sentencing Commission began its mammoth task of overhauling the federal sentencing process, one fundamental question has haunted its work: Is the commission constitutional?

Now, nearly four years after Congress established the panel to bring "certainty and fairness" to the sentencing of those convicted of federal crimes, the day of reckoning is near.

Defendants charged with crimes committed after Nov. 1 -- the day the new federal sentencing guidelines took effect -- are filing challenges to the constitutionality of the panel, claiming that the commission violates the doctrine of separation of powers and therefore the guidelines it wrote are void.

For the time being, the score is tied at three-all, with three federal judges ruling that the commission is constitutional; three more finding the other way (two in actual rulings, and one in comments to reporters indicating his eventual decision). More decisions are expected any day.

While the action so far has been in San Diego, where the federal public defender's office has been aggressively attacking the new sentencing rules, the epidemic of challenges has started to spread. A list compiled by the Justice Department notes 17 states in which defendants who fear their sentences will be longer under the new regime are claiming the guidelines are unconstitutional.

"It's just going on all over," said Patti Goldman of Public Citizen Litigation Group, which challenged the guidelines in a civil suit in federal court here and is participating in other cases by criminal defendants. "Every day we're getting a call from somebody saying, 'We're raising this challenge to the rules here.' "

In the meantime, she said, "It's chaos." Criminal defense lawyers are "paralyzed. They don't know what to do on behalf of their clients until they know what rules apply." Although the constitutional question has been looming since Congress passed the law, there was no possibility of a court decision until the guidelines took effect.

Braced for the attacks, the Justice Department has prepared a "contingency plan" for its prosecutors, advising them to appeal any rulings finding the guidelines unconstititutional and to continue to apply the new rules.

"This is an expected development," said Mark Robinson, special counsel to Assistant Attorney General William F. Weld, who disputed the characterization of the situation as chaotic. "Nobody from the beginning ever expected that the statute would not be challenged, and perhaps challenged successfully in the district court or even in the court of appeals. Until it does, there's going to be a period of some uncertainty."

The guidelines represent a sweeping change in sentencing for those convicted of federal crimes. Under the old system, judges enjoyed wide discretion in determining what sentences to impose, a situation that resulted in wide disparities. Because of the use of parole and "good time" credits, the actual time served often bore little resemblance to the sentence imposed.

The new rules strive to make the process more scientific, with an "offense level" assigned to each crime and a mathematical formula providing for various adjustments, up and down, based on other enumerated factors. Parole is eliminated, meaning that the sentence more closely reflects the time actually served.

The potential problem with the new regime -- constitutionally speaking -- is not with the guidelines but with the body that wrote them. Rather than tinker with the intricacies of the overhaul, Congress delegated the task to the newly created Sentencing Commission, required that three federal judges sit on the seven-member body, and characterized the panel as part of the judicial branch.

Critics, such as Alan B. Morrison of Public Citizen Litigation Group, point to a number of constitutional pitfalls with that approach.

First, they assert that the commission is unconstitutional because Congress delegated too much of its authority to make law to the unelected body. Second, they argue, even if Congress could delegate its power, it could not assign it to a body that is part of the judicial branch. Finally, they say, the provision giving the president power to remove members of the commission, including judges, further violates the doctrine of separation of powers.

Both the Department of Justice and the Sentencing Commission have come to the defense of the guidelines, but they are taking different approaches.

The Justice Department agrees with Morrison that the placement of the commission in the judicial branch -- with its "subservience to the president, its nonjudicial membership, and its functions outside the context of cases or controversies" -- would violate the separation of powers.

But, the department contends in a brief filed in the San Diego cases, Congress "simply mislabeled the commission" in terminating it as an entity within the judicial branch."

Once the Sentencing Commission is relabeled as part of the executive branch, the department adds, it poses no constitutional problem, because Congress has "power to create the executive authority to flesh out the sentencing function."

For its part, the Sentencing Commission insists that Congress meant what it said when it described the panel as part of the judiciary, and that its placement in the judicial branch does not violate the separation of powers.

All sides agree that the issue will ultimately be addressed by the Supreme Court. But with 40,000 federal defendants sentenced annually, the longer the question remains open, the more defendants who ultimately may have to be resentenced under the new or old regimes, depending on which way the high court goes.

If the court ultimately strikes down the guidelines, Congress could cure the problem by enacting them into law, but that route opens a potential Pandora's box of side issues, including reinstituting the death penalty for federal crimes.

At a meeting yesterday, the Sentencing Commission voted unanimously to urge the Justice Department to seek an expedited resolution of the constitutional issue.

"I'm sure it is causing problems because of uncertainty," said the commission chairman, Judge William W. Wilkins Jr. of the 4th U.S. Circuit Court of Appeals. "It simply means that cases will be handled initially differently depending on how that particular judge has viewed that legislation."

He said, "This is no different than what we have with any other piece of legislation being tested in the courts . . . . This was inevitable."