Nearly a quarter of all gun-related murders are committed by people ages 18 to 20, and that age group also is more likely than any other to use firearms in crimes that don't result in deaths, according to a new Justice Department report.

Vice President Gore will release the report Monday and call for the House to pass tough new guns laws during a speech before the 67th annual U.S. Conference of Mayors national meeting here.

With the future of a major gun-control proposal in the House increasingly uncertain, Democrats--and perhaps some moderate Republicans--will use the report to bolster the argument that the legal age to possess handguns should be raised from 18 to 21.

Many Republicans have opposed such a provision by, among other things, citing the incongruity of laws that allow 18-year-olds to fight in wars but not own guns.

In a letter to Gore, Deputy Attorney General Eric H. Holder Jr. and Treasury Undersecretary James E. Johnson said the report was significant because while so much political attention has been focused on rising crime among those under 18, the crux of the problem is among those a little older.

The report suggests that while 18- to 20-year-olds make up only 4 percent of the population, they commit about 24 percent of gun-related homicides. Holder and Johnson endorsed a number of provisions being considered in the House, including restricting the possession of handguns by anyone younger than 21 and increasing penalties for illegally selling firearms to youths.

"If possession of and access to firearms among this age group are reduced, deaths from armed crime will likely be reduced," Holder and Johnson wrote. "It is also reasonable to expect that by reducing access to firearms by this age group, it will also make it more difficult for their younger siblings, classmates and friends to obtain guns illegally, reducing gun violence among juveniles under 18."

Gun control and violence have been major themes during the five-day mayors' conference, which began Friday. A conference task force on gun violence has been gathering signatures from Democratic and Republican mayors this weekend for a letter to House Speaker J. Dennis Hastert (R-Ill.) urging the House to pass pending gun control legislation.

The gathering will provide a major forum for Gore to highlight an issue that Democrats are increasingly convinced will be a potent issue for them in the current election cycle--particularly if Republicans defeat the legislation in the House.

The conference also provided what Gore's aides suggested was further proof that his campaign, despite its early troubles and gaffes by the vice president, is starting to coalesce.

Early this afternoon, 167 Democratic mayors from around the country endorsed Gore's candidacy. Those signing on included Washington Mayor Anthony A. Williams and Baltimore Mayor Kurt L. Schmoke. Mayors of big cities such as Chicago, Miami-Dade County, Atlanta, Boston, Cleveland and Minneapolis as well as smaller towns, such as Capitol Heights and Hobart, Ind., also are supporting Gore.

Gore will begin his day here with a $1,000-a-plate fund-raiser at the Storyville Jazz Bar on New Orleans' famous Bourbon Street. Gore campaign officials said today that Louisiana Sens. Mary Landrieu and John Breaux as well as Mayor Marc Morial are helping to organize the event, which is expected to rake in as much as a quarter of a million dollars.

"We stand up early and we're going to stand up often," Morial said at a news conference attended by a couple dozen of the mayors. "We are here to say that we are going to make Al Gore the next president of the United States."

If the weekend belonged to Texas Gov. George W. Bush, who traveled to Iowa for his first campaign trip, then the Gore team wants the week to belong to him. He will officially announce his candidacy for president in Carthage, Tenn., on Wednesday.

The White House has clearly shifted into campaign mode, as was demonstrated by the fact that the Justice Department report was released by Gore's office along with a news release headlined, "Vice President Gore releases new study showing high rate of gun violence among American teenagers."

CAPTION: At a Thursday news conference, Rep. Edward J. Markey (D-Mass.), holding a report on youth guns, and other representatives pressed for anti-gun legislation to be brought to the House floor for a vote.