The nation's murder toll is headed toward a record high this year, with an 8 percent expected increase in the number of slayings that is attributable mainly to drug-related crime, the Senate was told today. ``If this pace continues _ and there is every reason to believe it will _ 1990 will be the bloodiest year in American history,'' said Senate Judiciary Committee Chairman Joseph R. Biden, D-Del. The committee issued a forecast of 23,220 murders in America this year at the present rate. That is 2,000 or 8 percent over the 1989 toll. The total would also top the previous record high of 23,040 murders in 1980. Scarce supplies of cocaine in the nation's major cities have contributed to the carnage, causing drug pushers to fight each other for turf, the committee said. It also blamed the proliferation of assault rifles. ``These firearms have become the weapons of choice for drug dealers and the weapons of doom for law enforcement personnel,'' said a report prepared by the panel. It said a third cause is a fresh wave of teen-agers, the offspring of the so-called baby boomers, entering the high-crime years. The projected toll would make 1990 the third straight year of increasing murders in the United States. The murder toll actually dropped 18 percent between 1980 and 1985 but has increased 22 percent since then, the committee said. ``The nation is faced with an immediate peril and the situation is doomed to get worse unless we take action today,'' Biden said. He urged swift House action on a Senate-passed omnibus crime bill with a ban on nine semiautomatic assault weapons and the death penalty for 34 federal offenses. A hush fell over the hearing room as Dr. Lynn Richardson, associate chief of emergency services at Harlem Hospital Center in New York, told of a young woman rushed in several weeks ago. The right side of her head had been blown away by a slug from a high-powered rifle in an apparent drug-corner shootout. Doctors managed to save a 6 1@2-month-old fetus, ``the ultimate innocent bystander,'' she said. The woman died but the child is still clinging to life, she said.