Attorney General John D. Ashcroft yesterday issued new guidelines to the nation's U.S. attorneys, requiring that they pursue the toughest charges they can reasonably hope to prove in criminal cases and limit their use of plea bargains.

The changes, combined with the attorney general's efforts earlier this year to seek the stiffest sentences possible, are intended to make the enforcement of criminal laws more uniform across the country. But they are seen by some prosecutors and judges as a serious erosion of their discretion in criminal cases.

"Like federal judges, federal prosecutors nationwide have an obligation to be fair, uniform and tough," Ashcroft said in a speech in Cincinnati yesterday. "It is the policy of the Department of Justice that federal prosecutors must charge and pursue the most serious, readily provable offense or offenses that are supported by the facts of the case except in limited, narrow circumstances."

In a memo to the nation's 93 U.S. attorneys, Ashcroft added that "just as the sentence a defendant receives should not depend upon which particular judge presides over the case, so too the charges a defendant faces should not depend upon the particular prosecutor assigned to handle the case."

The exceptions in the new guidelines include cases in which prosecutors believe they can obtain important evidence or information by plea bargaining with a suspect; cases in which "fast track" prosecutions can speed up clogged dockets and cases in which multiple charges would not affect the sentence.

The new guidelines were developed by an advisory committee of 15 U.S. attorneys. They represent a return to policies developed under the tenure of Attorney General Richard L. Thornburgh, who worked for President George H.W. Bush. The guidelines were relaxed during the Clinton administration under Attorney General Janet Reno, who gave federal prosecutors more discretion in filing charges.

The order comes on the heels of a new law that limits some judicial discretion in sentencing as well as an Ashcroft memo to U.S. attorneys in July ordering them to report to the department instances in which federal judges impose sentences lighter than those called for under federal sentencing guidelines. Ashcroft also has sought more uniformity in the imposition of the death penalty, at times overruling some U.S. attorneys' decisions not to pursue capital charges.

The effort to bind judges to the guidelines, supported by President Bush, is opposed by many jurists, among them Chief Justice William H. Rehnquist, who say Congress and the executive branch are trampling the independence of the judiciary.

Ashcroft's new guidelines on charging are likely to find the same opponents, along with some prosecutors who face curbs on their discretion in how hard to pursue cases.

Malcolm Young, executive director of the Sentencing Project, said the changes fail to recognize that "different cases are more or less provable than others.

"There is a uniform harshness in federal sentencing already," he added. "For the attorney general to say we have to tighten up and make sentences tougher just does not reflect the reality of the federal system today that imposes long sentences on many offenders."

Defense lawyer and former Justice Department prosecutor Victoria Toensing agreed, saying that "I think it is a very dangerous approach in light of the congressional action in taking power away from the judiciary."

But the exceptions to the new charging policy appear to leave prosecutors some of the same leeway they now enjoy. According to Justice Department statistics for fiscal 2001, more than 96 percent of criminal defendants pleaded guilty to the offense charged or to a reduced charge, or had their cases dismissed.

Prosecutors must be "fair, uniform and tough," Attorney General John D. Ashcroft said.