WASHINGTON, Dec. 11  The United States Sentencing Commission voted unanimously today to lighten punishments retroactively for crimes related to crack cocaine, a decision that could affect some 19,500 federal inmates.

The decision, which was made over the objections of the Bush administration, takes effect on March 3, 2008, and it will not mean automatic release for those serving time. But it does open the door for them to apply for sentence adjustments and possible earlier release.

The commission previously had voted to recommend shorter prison sentences for crack cocaine offenders. Those guidelines went into effect on Nov. 1, but the commission postponed a vote on whether to make them retroactive.

The commission’s vote addressed what many analysts have come to see as a deeply unfair consequence of the decades-long war on drugs and the drug-sentencing laws adopted in the mid-1980s, when the fear of drug-induced street violence had a profound effect on public policy and personal behavior. A 2002 commission report noted that 85 percent of defendants convicted of crack offenses were black, a fact the commission warned was leading to a loss of confidence in the fairness of the justice system.