Standing between D.C. Mayor Marion Barry and any talk of plea bargains in his drug conspiracy case, and looming over the prospect of a trial, is a collection of charts and graphs in an inch-thick blue book: the U.S. Sentencing Guidelines.

The term guidelines is a misnomer. In fact, they are a blueprint judges must now use in computing sentences. They severely restrict a judge's broad sentencing discretion, and they limit the ways in which it works to the defendant's advantage to enter a plea instead of going to trial.

Since the guidelines went into effect in November 1987, both aspects have created major changes in the way criminal trials are conducted in federal courts.

The most important result is in plea bargaining. Nowadays, the only way in which plea bargains can greatly work to a defendant's advantage is if the defendant agrees to give the government "substantial assistance" in developing another criminal case.

That, say sources close to Barry, is something the mayor would not do. And because the sentence computation would change only slightly if the mayor entered a plea, the guidelines give him little incentive to do so.

Sources close to U.S. Attorney Jay B. Stephens have said that he would accept nothing less than a guilty plea to one of the felony perjury counts in the 14-count indictment. That would result in at least a six-month sentence for the mayor. Here's how:

The guidelines assign a "base offense level" number to every federal crime. They then refer lawyers to a variety of aggravating and mitigating factors that may raise or lower that base level. The offense level then keys in to a chart spelling out the sentencing range in months.

If Barry were to follow the scenario hinted at by Stephens, he would start with a base offense level of 12. That could be reduced by two by U.S. District Judge Thomas Penfield Jackson if the judge interprets his plea as an "acceptance of responsibility" under the guidelines.

Or the judge could refuse to do that and raise the offense level by three -- to 15 -- if he thought Barry's conduct fits a provision of the guidelines calling for a more severe penalty if the perjury "resulted in substantial interference with the administration of justice." That is defined in part as "the unnecessary expenditure of substantial governmental or court resources."

A base offense level of 10 carries a prison term of six months to one year. A 15 calls for a term of 18 months to two years. If the sentence were six months, Jackson could allow Barry to serve it on probation or he could sentence him to a halfway house.

Politically, the consequences of pleading guilty to a felony would be far worse. D.C. Corporation Counsel spokesman Claude Bailey said a D.C. mayor must be a "qualified elector" -- and a felony conviction that carries a prison term removes that qualification.

Bailey said it is an open question whether the mayor could hold office if convicted of a felony and given probation. City law says felons who have completed their jail term can hold office, he said, but whether probation fits that description is unclear.

If Barry goes to trial and is convicted on all counts, or any combination of counts, the guidelines call for the charges to be telescoped into units. A formula is used to yield a base offense level for the entire case -- in this instance, 13.

For a first offender, that offense level calls for a prison sentence of 12 to 18 months, and the time must be served in prison.

If the mayor is convicted only of perjury, and his base offense level doesn't change with any aggravating or mitigating factors, he would wind up with a 10-to-16-month sentencing range. In that case, Jackson could give Barry the minimum and allow him to serve half of the 10 months on probation.

If Barry is convicted only of misdemeanor counts, the guidelines call for a sentence ranging from probation to six months. In practice, that would almost certainly mean probation. A misdemeanor conviction -- even one that carried a short jail term -- would be no barrier to Barry's continuing to hold office, Bailey said.

One last caveat: Jackson can ignore the guidelines and allow the mayor to serve any sentence on probation, but that would not be easy. The judge would have to come up with an extraordinary reason not encompassed in the guidelines, and his decision would be grounds for government appeal.