Momentum is growing for legislation this year to narrow the sentencing disparity that keeps defendants convicted of trafficking crack in jail far longer than for powdered cocaine.

African-American lawmakers and liberal groups have long decried the fact that it takes 100 times more powdered cocaine than crack to trigger lengthy mandatory minimum prison sentences, a disparity that disproportionately affects minority defendants.

A combination of factors — including the Democratic takeover of Congress and growing Republican interest in alternatives to lengthy prison sentences — is helping create momentum for narrowing the differential first enacted 21 years ago.

“We’re starting off where there’s relatively broad support for some type of reform coming from all parts of the political spectrum,” said Marc Mauer of the Sentencing Project, an advocacy group that has argued for lowering prison sentences.

Last week, Jeff Sessions, R-Ala., one of the most conservative members of the Senate Judiciary Committee, announced plans to introduce legislation that would raise the quantity of crack and lower the quantity of cocaine that would trigger a mandatory minimum sentence so that the ratio would be 20-1, rather than 100-1.

Sen. Orrin G. Hatch, R-Utah, is expected to introduce an alternative that would reduce the ratio to 20-1 by only increasing the crack trigger, an approach favored by liberal groups that don’t want any changes to result in lengthier sentences for cocaine defendants.

While the Senate Judiciary Committee has not settled on a single approach, Hill staffers and lobbyists for several groups expect Joseph R. Biden Jr., D-Del., chairman of the Judiciary Subcommittee on Crime and Drugs, to introduce legislation in coming weeks that would set the ratio lower than 20-1.

The sticking point may be on the House side, where members of the Congressional Black Caucus, including Robert C. Scott, D-Va., chairman of the House Judiciary Subcommittee on Crime, Terrorism and Homeland Security, have long insisted that the triggers for crack and cocaine be equalized.

“The facts haven’t changed,” Scott said. “There’s no justification for any differential.”

Charles B. Rangel, D-N.Y., has introduced a bill (HR 460) that would make them equal.

But even if House Democrats compromise, getting a bill passed in that chamber might still be difficult, says one Republican staff member familiar with the issue.

“I don’t think the Democrats are going to push this issue coming into 2008 unless they get a lot of Republicans on board, and I don’t think they will,” the staff member said.

There is disagreement over the impact of a proposed change in sentencing guidelines the U.S. Sentencing Commission unveiled May 1.

Advocates for narrowing the crack-cocaine disparity praised the proposal, which would more closely align the sentencing guidelines with mandatory minimum sentences passed by Congress. The change could reduce crack sentences by about 16 months on average.

But the Sentencing Commission rankled House members who believe it is Congress’ job to make changes in sentences. The proposal, which goes into effect in November unless Congress intervenes, could also provide political cover for Republicans to argue that it negates the need for Congress to intervene.

The Sentencing Commission also released its latest report on federal cocaine sentencing policy, recommending the elimination of the mandatory minimum prison sentence for simple possession of crack, which is the only drug that triggers prison time for mere possession.

Patrick J. Leahy, D-Vt., chairman of the Senate Judiciary Committee, called the report “an important first step in correcting the wide disparity in our federal sentencing laws.”

It was the fourth such report by the Sentencing Commission on the issue in the last 12 years, many of which have recommended narrowing the gap. But until this year, the commission has proved gun-shy about offering amendments to sentencing guidelines since Congress rejected its 1995 amendment to equalize crack-cocaine sentences.

The Bush administration has maintained that it prefers to consider the crack-cocaine disparity as part of a broader review of sentencing policy. That would probably include an effort to curb the discretion the Supreme Court gave federal judges when it ruled that the federal sentencing guidelines are advisory rather than mandatory.

Groups that have long advocated for narrowing the disparity say they would prefer parity but would settle for something less.

“We’re willing to consider legislation that would make a real difference in these sentences,” said Mary Price, general counsel of Families Against Mandatory Minimums.

Still, both advocacy groups and legal experts say Congress might be better off abandoning the emphasis on quantity triggers entirely and adopting an approach that focuses on the culpability of defendants such as drug kingpins and middlemen.

Under current law, possession of five grams of crack, equivalent to the weight of two sugar packets, triggers a five-year mandatory minimum prison sentence.

“There would be greater potential for political harmony if someone could move the debate away from quantities toward more sensible substantive concerns such as making sure kingpins get harsh sentences,” said Douglas A. Berman, an Ohio State University law professor who tracks sentencing laws.