Whether Jackson has the discretion to ignore the guidelines is the issue before the Supreme Court on Tuesday.
A companion case from Iowa also involves a judge's discretion to impose a more lenient sentence in a drug case, although Brian Gall pleaded guilty to conspiracy to distribute ecstasy. In Gall's case, the judge decided probation was sufficient punishment even though the guidelines called for prison time.
Federal appeals courts threw out both sentences, but the justices accepted the defendants' appeals.
The Bush administration is supporting the appeals court rulings, while civil rights and advocacy groups are backing the defendants.
Congressional opponents of the laws establishing more severe sentencing for crack cocaine than powdered cocaine are racially discriminatory because they hit more directly at the black community, where this form of drug abuse is more commonplace.
Advocates for reducing the disparity point to crime statistics that show crack is more of an urban and minority drug while cocaine powder is used more often by the affluent. They say harsher penalties for crack cocaine unfairly punish blacks.
More than four-fifths of crack cocaine offenders in federal courts last year were black, according to the U.S. Sentencing Commission. By contrast, just over a quarter of those convicted of powder cocaine crimes last year were black, the commission said.
Kimbrough actually had much more powder than crack, but it was the latter that determined the length of his prison term.
The 4th U.S. Circuit Court of Appeals in Richmond said judges are not free to impose sentences shorter than the guidelines "based on a disagreement with the sentencing disparity for crack and powder cocaine offenses."
The crack-powder disparity grew out of a 1986 law that was passed in response to violent crimes committed to get money to feed crack habits.
The law includes what critics have called the 100-to-1 disparity: Trafficking in 5 grams of crack cocaine carries a mandatory five-year prison sentence, but it takes 500 grams of cocaine powder to warrant the same sentence.
The U.S. Sentencing Commission, an independent agency within the U.S. judiciary, voted in May to reduce the recommended sentencing ranges for people convicted of crack possession, a step toward lessening the disparity. The recommendation will become effective Nov. 1 unless Congress acts.
At the same time, the commission urged Congress to repeal the mandatory prison term for simple possession and increase the amount of crack required to trigger obligatory five-year or more prison terms as a way to focus on major drug traffickers.
The Supreme Court gave a boost to judges' discretion when it ruled in 2005 that the sentencing guidelines are advisory, not mandatory. The guidelines were adopted in the 1980s to ensure comparable sentences for similar crimes from courtroom to courtroom.