With the District's new mandatory sentencing law not expected to take effect until next year, possibly as late as March, leaders of the successful campaign for the tough new law say they will propose an advertising campaign to educate the public about the measure.

The law, approved by D.C. voters in the Sept. 14 primary election, requires mandatory prison terms for crimes committed with a gun, and for certain drug offenses. Legislative aides said last week that it could be March before the measure takes effect, because of the required congressional review period.

In the meantime, D.C. City Council member John Ray (D-At large), who led the campaign for passage of the mandatory sentencing initiative, said that he plans to propose legislation to pay for advertisements to educate the public -- and potential offenders -- about the penalties set forth in the new law.

Ray said that advertising, whether on television or in storefront windows, "is very important to get the full impact" of the new law.

Officials in other states with mandatory sentencing laws have credited advertising with helping to reduce targeted crimes, a result they say they saw in crime statistics at least during the first year fixed penalties were in effect.

Ray said he also expects that Citizens for Safer Streets, a group he founded to lobby for the sentencing initiative, will stay in business even after the measure becomes law.

The Board of Elections last week certified that the proposed new law, known on the ballot as Initiative 9, was approved by an overwhelming majority of D.C. voters. According to the board, 84,012 voted for the measure and 32,333 voted against. The board forwarded a copy of the new law to City Council Chairman Arrington Dixon, who must now transmit it to Congress, where all new District legislation must be reviewed before it becomes law.

The new law will require local judges to impose a mandatory, five-year prison term for first offenders convicted of committing a violent crime with a gun and a 10-year term for second offenders. Mandatory penalties for drug offenses range from 20 months to four years, depending on the type and quantity of drugs sold.

The law states that there will be no opportunity for parole, probation of other form of release until the mandatory minimum prison term is served in full.

Most observers are confident that Congress eventually will approve the measure without alteration.

"I would be appalled if they attempted to change it," Ray said last week. "This is a local initiative. If there are any changes to be made, it's up to the City Council."

City legislators and local prosecutors say privately that they believe it would be politically unwise for Congress to oppose a law so clearly intended as an effort to get tough on crime in the nation's capital.

As things now stand, Congress will not be returning to Capitol Hill from its election recess until Nov. 29. Congressional and City Council aides interviewed last week said they doubt that the 30-day review period, which counts only days on which either of the two houses is in session, can be completed before the Christmas recess. This means that the bill would be reintroduced early in January with the start of the 98th Congress, at which point the 30-day period would begin again.

Bruce French, legislative counsel to the D.C. City Council, said last week that Initiative 9 probably would become law some time in mid-March. French said he bases this projection on the experience of previous initiatives approved in the fall that had to be retransmitted to Congress after the start of the new year because the 30-day review period ran short.

For example, French said, the statehood initiative, which set in motion the city's constitutional convention, and the gambling initiative, which authorized a city-run lottery, were approved by voters in the November 1980 election but went into effect on March 10, 1981, after the 30-day congressional review period.

The 1973 Home Rule Act permits either the House or the Senate to disapprove of a new D.C. law that alters the criminal code, provided that Congress takes such action within 30 legislative days after the measure is sent to it from the City Council.