years to defendants' prison sentences, under the government's 17-year-old system, which has been challenged as unconstitutional.

Justice Antonin Scalia said that lucky accused criminals go before merciful judges. The unlucky, he said, can face a "hanging judge."

"The whole reason for jury trials is we don't trust judges," he said.

Scalia wrote a stunning 5-4 rul ing in June that struck down a state sentencing system because it gave judges too much leeway in sentencing. The ruling caused judges around the country to delay sentencings

or hand down lighter penalties.

Justices heard two follow-up appeals Monday, on the opening day of their new term, that will decide the fate of the federal system set up by Congress to make sentencing more fair.

While juries consider guilt or innocence, judges make factual decisions that affect prison time, such as the amount of drugs involved in a crime, the number of victims in a fraud or whether a defendant committed perjury during trial.

The three-month summer break did not appear to erase the worries of five justices that a defendant's Sixth Amendment right to a jury trial is undermined when a judge, instead of jurors, makes critical decisions that add time to the sentence.

Instead, justices wrangled for nearly two hours over what parts of the sentencing scheme are acceptable, as well as the practical impact on jury trials of their eventual decision.

"Maybe we should just leave it to Congress to decide," said Justice Sandra Day O'Connor, who had said in a speech this summer that justices caused a "No. 10 earthquake" in courthouses with the June ruling.

The federal guidelines were being challenged in two cases involving men convicted on drug charges. Guidelines, used in federal courts and by many states, give judges a range of possible sentences for each crime, but factors affect the final sentence.

Justice Anthony M. Kennedy said that without guidelines, judges have "unexplained, unarticulated, standardless discretion."

"You don't have to throw out 20 years of sentencing reform," said Rosemary Scapicchio, a Boston attorney representing a man in one of the two cases.

She said the only change is that jurors, not judges, will handle those decisions.

But Paul Clement, the Bush administration's acting solicitor general, said that juries' jobs will become too complicated. They will face pages and pages of questions, instead of straightforward decisions about someone's innocence.

"We give jury instructions that are complicated all the time," said Christopher Kelly of Madison, Wis., the attorney for the other man.

After the court's June ruling, federal prosecutors changed the way they indict defendants and handle plea bargains. They've also been asking judges to consider two sentences -- one to be used should the sentencing structure be found constitutional, and one if it were overturned.

Monday's appeals involve people sentenced on drug charges in Wisconsin and Maine. Both will get lighter sentences if the court rules against the government, as could other defendants whose sentences are not final.

"They are seeking a huge sentencing windfall here," Clement said.

Clement told the justices they don't have to throw out the entire sentencing law. A ruling is likely before the end of the year.

Attorney General John Ashcroft said at a Monday news conference that crime has dropped and tough sentencing guidelines are one key reason.

"If you do the crime, you'll do the time," he said in urging justices to keep the current system. "It is profoundly essential that we have certainty of punishment.

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