House and Senate conferees opened the door yesterday to the imposition of tough mandatory minimum sentences for drug traffickers in the District, as they completed work on a $3.8 billion spending bill for the D.C. government for fiscal 1991.

The conferees also approved an additional $30 million in federal funds for drug treatment, schools and other purposes, while deleting a provision from the appropriations bill that would permit the local Big Brothers organization to bar gay men from serving as mentors to youths.

Under a proposal by Sen. Phil Gramm (R-Tex.), the D.C. Criminal Code would be amended to provide for progressively stiffer mandatory sentences, beginning with a 10-year sentence for people possessing a firearm while commiting a violent or drug trafficking crime.

Those convicted of third offenses would be subject to sentences of life in prison without parole, according to the proposal.

The Senate, acting in response to the escalating number of homicides in the District, approved the mandatory sentences last month as part of its version of the D.C. appropriations bill. The provisions, which are opposed by the District government and some key congressional Democrats as an intrusion on home rule, are not included in the House version.

Neither side would give way at yesterday's conference committee markup, necessitating a vote on the measure in each house next week. In light of the get-tough-on-crime sentiment in Congress, legislators and staff members from both parties predicted the mandatory sentencing measure would gain approval in the House and Senate.

Sen. Robert C. Byrd (D-W.Va), chairman of the Appropriations Committee, voted by proxy to support the sentencing measure.

"We have an excellent opportunity to prevail," said Gramm, the ranking Republican on the Senate Appropriations subcommittee on D.C. "Congress feels very strongly about the crime wave sweeping the District."

District officials expressed disappointment that the Gramm amendment survived the conference meeting, pointing out that the D.C. Council has already moved to adopt mandatory minimum sentencing.

The D.C. Council in 1989 approved a law imposing five-year minimum sentences for anyone convicted of possessing a firearm during the commission of a felony.

In 1983, the city enacted its first mandatory sentencing law that provided mandatory minimum prison sentences with no chance for parole for persons found guilty of certain drug offenses or committing violent crimes with guns.

"We believe that the District government has acted responsibly in increasing the punishment for drug-related crimes and violent crimes," said Garland Pinkston, director of the mayor's Office of Intergovernmental Relations.

"We're hopeful that Congress will leave this as a local prerogative," he said.

The bill approved by the conferees would restore the District's right to fund abortions for poor women -- a right taken away by Congress two years ago.

But in a letter to congressional leaders this week, Richard G. Darman, director of the Office of Management and Budget, signaled President Bush's intention to veto the appropriations bill if it contains money for abortions, as the president did twice a year ago before Congress deleted the funds.

Darman said in the letter that the president's senior advisers would recommend that Bush veto the bill if it permits local funding of abortions, "except where the life of the mother would be endangered if the fetus were carried to term."

Rep. Julian C. Dixon (D-Calif.), chairman of the House Appropriations subcommittee on D.C., suggested that the congressional leadership might try to attach the District appropriations bill to other spending bills that Bush might be more reluctant to veto as Congress scrambles to adopt a budget in the closing days of its session.

"The president has done so many strange things recently," Dixon said. "I don't know what his attitude would be."

A spokeswoman for Sen. William L. Armstrong (R-Colo.), sponsor of the Big Brothers amendment, hinted that Armstrong might stage a floor fight to restore the language excised by the conferees.

Armstrong's amendment was designed to scuttle an agreement last year by the local Big Brothers group to allow gay men to serve as mentors to children if their parents approve.

While defeating the Armstrong amendment, the House-Senate conferees also:

Reinstated the congressional prohibition on removing the suicide prevention barriers at the Ellington Bridge in Northwest Washington, saying the city could not remove the barriers without holding an evidentiary hearing.

Approved an amendment prohibiting the city from spending any public funds on the shadow senator and representative positions designed to lobby for D.C. statehood.

Prohibited the city from occupying the Hurt Home for mentally troubled youths in Georgetown until a lawsuit by neighbors against its use is litigated in the courts.

The home is currently being renovated and is not ready for to be occupied, congressional officials said.