WASHINGTON -The Bush administration warned the Supreme Court yesterday against inflicting “carnage” and “wreckage” on the federal criminal justice system by finding that two decades-old federal sentencing guidelines are unconstitutional.

Convicts would enjoy a “huge sentencing windfall” if judges are barred from increasing sentences on the basis of factors that had not been proven before a jury, argued the acting solicitor general, Paul Clement.

The court opened its new term with an expedited oral hearing aimed at addressing the confusion created last June when the justices struck down Washington State sentencing rules that are similar to the federal guidelines because they allow judges to make findings of fact that increase penalties.

The decision could affect potentially thousands of criminal sentences and transform the federal rules designed to reduce disparities in criminal sentences.

The five judges who made up the majority in the June decision known as Blakely gave no indication they would treat federal guidelines differently from state rules, despite Mr. Clement’s argument that federal rules were not set by a legislature but by a judicial sentencing commission.

Justice Scalia said the Sixth Amendment’s guarantee of a right to a jury trial was intended “precisely” to protect accused criminals from fact-finding by judges.

“Who are you worried about when you say you want to be tried by a jury? You’re worried about the judge, not the legislature. …The whole reason for a jury trial is because we don’t trust judges,” he said.

Justice Souter, another member of the Blakely majority, also questioned the difference between the state rule and the federal guideline. “If it’s a rule, a guideline, or a statute, why should it make any difference?”

Mr. Clement told the court that requiring all factors be proven before a jury would result in lengthy indictments and complex jury instructions that would leave juries “completely bewildered.”

But Justice Souter downplayed the concern, asking, “Surely the argument can’t be that just because it’s more complicated, that the Sixth Amendment argument evaporates?”

Mr. Clement estimated the changes would affect 65% of all federal criminal sentences, but Justice Stevens, another member of the Blakely majority, estimated the changes would affect only a fraction of the 3% of federal criminal cases that do not result in plea bargains.

Mr. Clement also argued that putting all aggravating factors to a jury would make the prosecution of certain kinds of cases difficult, such as telemarketing fraud involving thousands of victims whose individual circumstances could not be proven to a jury beyond reasonable doubt.

Justice Scalia interjected that if the charges could not be proven to the jury “then maybe judges shouldn’t be guessing” how many people were involved.

Mr. Clement also warned that certain factors such as perjury only emerge as a trial unfolds, and could not be part of an indictment.

But Justice Ginsburg, who was also part of the Blakely majority, questioned whether such problems are “intractable.” Such issues are often tried separately, several judges noted.

Much of the argument centered on what kind of scheme should replace the guidelines, if they are struck down.

Justice O’Connor, who was in the minority in Blakely, questioned suggestions that the guidelines be made optional, explaining that Congress had intended them to be mandatory restraints that equalize sentences.

Judge Scalia predicted that any solution crafted by the court would be an “interim solution” until Congress could weigh in.

“Why can’t we say that when these facts are not found by a jury, the sentences are unconstitutional, and let the government work out how it finds its way around the problem,” he said.

The court is considering the case of Freddie Booker, a Wisconsin man who was charged and convicted by a jury of possession of 50 grams of cocaine, after the government chose not to charge him with possession of 658.5 grams of cocaine that was found at his arrest. A Wisconsin judge later held him responsible for the full amount when he sentenced him to a prison term of 360 months. An appeals court said Booker should be resentenced because the aggravating facts had not been proven to a jury.

In the other case before the court, a Massachusetts man, Duncan Fanfan, was convicted by a jury of possessing with the intent to distribute at least 500 grams of cocaine. A judge then held him responsible for 2.5 kilograms of cocaine and organizing criminal activity, sentencing him to a term of 188 to 235 months. Applying the Blakely decision, an appeals court reduced the sentence to 78 months because the aggravating factors had not been found by the jury.