A diverse coalition of children's rights, civil liberties and states' organizations is vigorously lobbying against Republican legislation aimed at cracking down on youth crime. Even Supreme Court Chief Justice William H. Rehnquist has raised objections to the bill. But Senate GOP leaders, insisting public support is on their side, are vowing the measure will be among the first considered this year. Given the looming elections, they seem likely to prevail.

The bill, the Violent and Repeat Juvenile Offender Act, would provide a total of $ 2.5 billion to states over five years for programs to rehabilitate juvenile offenders and prevent juvenile crime. Opponents, however, contend that the money comes at too high a cost. To receive the block grants, states would have to treat anyone age 14 and up accused of serious drug or violent crimes as an adult. The bill would also open the door to greater federal prosecution of juveniles and give jailers more leeway to house youth and adult prisoners together.

Last May, similar legislation passed the House, 286- 132. But the coalition of some 300 organizations, ranging from the Children's Defense Fund (CDF) to the American Civil Liberties Union to the National Governors' Association (NGA), has redoubled its efforts to block it in the Senate. The group last fall sent a letter laying out its concerns to the bill's chief Senate sponsor, Judiciary Committee chairman Orrin G. Hatch, R-Utah.

GOP lawmakers say opinion polls show the public backs adult penalties for youths who commit serious offenses. They say ''get tough on crime'' policies pushed by Republicans are responsible for the continuing decline in overall crime nationwide. Likewise, some criminologists say that separating out the most violent youth offenders may benefit juveniles convicted of lesser crimes. ''We're not going to let special interests stand in the way of the public interest when it comes to juvenile justice,'' said a Hatch spokeswoman.

The bill's supporters point to census data showing that as the children of baby-boomers mature, the teenage population is expected to grow 15 per cent over the next six years. Males in their teens, studies show, are disproportionately prone to commit crimes.

Some analysts note that in an election year, it's a sure bet that lawmakers will feel pressured to support a seemingly popular anti-crime measure. ''This will not be a year marked by the passage of many bills, but this bill should pass,'' said Norman J. Ornstein, a senior fellow at the American Enterprise Institute for Public Policy Research. He said the congressional debate would very likely coincide with the trial of Michael Carneal, a Kentucky 14-year-old charged with gunning down three classmates in December. He is being tried as an adult.

Still, opponents haven't shown any signs of letting up. ''No one supports this bill, '' contended Kim Wade, a lawyer with the CDF. Marc Schindler, an attorney at the Youth Law Center, which is aligned with the CDF, said recent studies show that juveniles incarcerated with adults are more likely to be beaten, be raped and commit suicide. By contrast, Schindler argued, young offenders housed in juvenile facilities receive the psychological attention and education they need to reenter society.

States' organizations such as the National Governors' Association, meanwhile, fear the legislation could create a record-keeping nightmare, because it would require states to maintain juvenile records for longer periods and to pass along many records to the FBI. ''States will have to jump through hoops to get the grant money,'' said NGA spokesman Nolan Jones.

For his part, Rehnquist in his recent year-end report warned of ''legislation pending in Congress to 'federalize' certain juvenile crimes.'' He argued that the bill could further overburden federal courts and that ''federal prosecutions should be limited to those offenses that cannot or should not be prosecuted in state courts.''

With pressure growing, Democratic Senators led by Judiciary Committee ranking member Patrick J. Leahy of Vermont hope that moderate Republicans can be persuaded to support changes to the legislation. Leahy previously pushed his own juvenile justice bill but is now working with a group of Democrats on amendments to the Hatch measure. According to Leahy aides, the Senators will try to soften the conditions under which states can receive grant money, restore restrictions on housing juvenile and adult offenders together, and provide more money for juvenile crime prevention.

Worried that Democratic Senators will also try to require firearm makers to equip guns with child-safety locks, the National Rifle Association (NRA), too, has begun lobbying against the bill.

President Clinton has pushed hard for the child-safety locks and after-school crime prevention programs. Last year, Clinton endorsed Leahy's bill. Now, the White House says Clinton will wait to see what changes are made to the juvenile justice bill in the Senate--or in a conference committee--before deciding whether to sign it. The White House is expected to work closely with Senate Democrats on their amendments.

The AEI's Ornstein said he expects Democrats to succeed in adding the child-safety locks. But he also predicted that stricter penalties for violent juveniles would pass since tougher punishments have, at the least, contributed to a drop in crime in both Boston and New York City. In the end, he said, ''Democrats are going to want to get behind the bill. ''

DATELINE: WASHINGTON

Sen. Edward Kennedy is trying to revive efforts to channel federal money into crime prevention for youths, four years after "midnight basketball " became a buzzword for Republican critics of such prevention efforts.

Kennedy's legislation, introduced Monday, would spend $ 7.25 billion over five years on after-school programs. The aim is to keep youths occupied - and away from trouble - between the crucial hours of 3 p.m. and 8 p.m.

"It's not 'Leave It To Beaver' anymore, June is not in the kitchen, " said Rep. Louise Slaughter, D-N.Y., who is sponsoring a companion measure in the House. "Children need some help. "

One component of the "America After-School Act " would provide $ 1.25 billion over five years in matching grants to local public and private agencies for after-school prevention programs in high-crime neighborhoods and areas with high numbers of youths considered at risk for committing crimes.

To fund the grants this year, Kennedy's bill would direct that half of $ 500 million already approved by Congress for juvenile justice law enforcement could be used for prevention activities.

Kennedy said that proposal has met with resistance from Republicans on the Judiciary Committee, chaired by Sen. Orrin Hatch, R-Utah.

"There will be a battle on the floor of the U.S. Senate, " he said.

Jeanne Lopatto, spokeswoman for the committee, said juvenile justice legislation now pending in committee already includes $ 1 billion over five years that state and local governments could use for prevention.

"It can be used for anything, " Lopatto said. "We feel that's the best way to go. "

Kennedy, who estimated that 5 million children - some as young as 8 or 9 - spend some time alone after school, favors linking federal funds more directly to crime prevention efforts.

Crime prevention programs are a Democratic priority often denigrated by Republicans, who undermined them after taking over Congress in 1995. Most prevention programs have been eliminated in favor of providing block grants with few strings attached to local communities.

Although the 1994 crime law, which spawned debate over the effectiveness of midnight basketball leagues and other efforts targeting at-risk youths, authorized $ 6.9 billion for crime prevention, only $ 688 million has been spent.

Kennedy's bill would also provide $ 5 billion over five years to expand state block grants to increase the availability and affordability of before- and after-school care for school-age children, and for weekend and summer activities. Kennedy has suggested funding this through a tobacco tax or by reducing corporate tax breaks.

Another $ 1 billion over five years would go toward helping public schools expand before- and after-school programs, such as by improving access to their libraries, gymnasiums and computer laboratories. This program is included in President Clinton's balanced budget proposal for 1999.

Kennedy and Sen. John Kerry, D-Mass., a co-sponsor, hope Congress takes a cue from Boston, where the city police department has developed a nationally renowned program for preventing juvenile crime.

The city went 29 months without a juvenile murder before Dec. 11, 1997, when 16-year-old Eric Paulding was shot to death in the city's Dorchester section.