WASHINGTON — Congress on Wednesday changed a quarter-century-old law that has subjected tens of thousands of blacks to long prison terms for crack cocaine convictions while giving far more lenient treatment to those, mainly whites, caught with the powder form of the drug.

The House, by voice vote, approved a bill reducing the disparities between mandatory crack and powder cocaine sentences, sending the measure to President Barack Obama for his signature. During his presidential campaign, Obama said that the wide gap in sentencing “cannot be justified and should be eliminated.”

The Senate passed the bill in March.

The measure changes a 1986 law, enacted at a time when crack cocaine use was rampant and considered a particularly violent drug, under which a person convicted of crack cocaine possession gets the same mandatory prison term as someone with 100 times the same amount of powder cocaine.

The legislation reduces that ratio to about 18-1.

The bill also eliminates the five-year mandatory minimum for first-time possession of crack, the first time since the Nixon administration that Congress has repealed a mandatory minimum sentence. It would not apply retroactively.

“For Congress to take a step toward saying ‘we have made a mistake’ and this sentence is too severe ... is really remarkable,” said Virginia Sloan, president of the Constitution Project. The group in studies of sentencing practices has referred to crack cocaine mandates as a “’poster child’ for the injustices of mandatory sentencing.”

Under current law, possession of five grams of crack triggers a mandatory minimum five-year prison sentence. The same mandatory sentence applies to a person convicted of trafficking 500 grams of powder cocaine.

The proposed legislation would apply the five-year term to someone with 28 grams, or an ounce, of crack.

Julie Stewart, president of Families Against Mandatory Minimums, said 28 grams is about what the average crack dealer might carry around.

She said politicians and the U.S. Sentencing Commission have for years acknowledged the unfairness of the system, “but no one wanted to look soft on crime.” The legislative change, she said, is “much more about being smart on crime.”

She cited Sentencing Commission estimates that almost 3,000 people a year subjected to the mandatory sentence would be affected by the change. The average sentence in these cases would be reduced from 106 months to 79 months.

Sen. Dick Durbin, D-Ill., the main sponsor of the bill in the Senate with Sen. Jeff Sessions, R-Ala., said last year close to 1,500 people were convicted for possession of somewhere between five and 25 grams of crack cocaine, subjecting them to mandatory minimum sentences.

Some 80 percent of those convicted of crack cocaine offenses are black.

In the 2008 campaign, Obama said the sentencing disparity “has disproportionately filled our prisons with young black and Latino drug users.” He cited figures that blacks serve almost as much time for drug offenses — 58.7 months — as whites do for violent offenses — 61.7 months.

The Congressional Budget Office said the bill would save the government $42 million over five years because of the reduction in prison populations.

Durbin said he voted for the harsh sentences when he was a House member in 1986. When crack first appeared on the scene, “there was near panic in the halls of Congress” over the new cheap, addictive and destructive drug. “It scared us to death. We overreacted.”

House Majority Leader Steny Hoyer, D-Md., a chief proponent in the House, also acknowledged that, in responding to the addictiveness of crack and the violence it brought, “Congress also created a wide disparity” between crack and powder cocaine sentences.

“The 100-1 ratio has caused myriad problems, including perpetuating racial disparities, wasting taxpayer money and targeting low-level offenders instead of dangerous criminals,” the Drug Policy Alliance said in a statement.

Rep. Lamar Smith of Texas, the top Republican on the Judiciary Committee, was the only lawmaker to speak against the bill, saying the 1986 law was enacted at a time when the crack cocaine epidemic was bringing a sharp spike in violence to minority communities and it would be a mistake to change it.

“Why do we want to risk another surge of addiction and violence by reducing penalties?” he asked. “Why are we coddling some of the most dangerous drug traffickers in America?”

Rep. Bobby Scott, D-Va., noted that the bill also requires the sentencing commission to significantly increase penalties for drug violations involving violence. “This way the defendant is sentenced for what he or she actually did, not the form of cocaine involved,” Scott said.

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Online:

Information on the bill, S. 1789, can be found at http://thomas.loc.gov

U.S. Sentencing Commission: http://www.ussc.gov/

Families Against Mandatory Minimums: http://www.famm.org/