Sen. Joseph R. Biden Jr. (D-Del.) said yesterday that he will head the Senate Foreign Relations Committee and a new Judiciary subcommittee on crime and drugs when Democrats take control of the Senate next week, averting a domino-like upheaval on several major panels.

Biden's decision, announced at a news conference in Wilmington, Del., paves the way for Sen. Patrick J. Leahy (D-Vt.) to head the Judiciary Committee, which will be at the frontline for action on President Bush's judicial nominations. It also ensures that Sen. Tom Harkin (D-Iowa), who faces a tough reelection campaign next year, will become chairman of the Agriculture Committee.

Biden, Leahy and Harkin are the ranking Democrats on their respective committees.

An internationalist with a wide-ranging interest in world affairs, Biden will succeed Sen. Jesse Helms (R-N.C.), whose six-year tenure as Foreign Relations chairman has been marked by skepticism of overseas entanglements.

Biden, 58, who has served in the Senate for 28 years, has more seniority than any other Democrat on both the Foreign Relations and Judiciary panels and could take the chairmanship of either. He headed Judiciary from 1987 to 1995 and has been the ranking Democrat on Foreign Relations since 1997.

Shortly after Vermont Sen. James M. Jeffords announced last week that he was leaving the Republican Party and becoming an independent, giving Democrats control of the Senate, Biden said he was undecided about which panel he wanted to head, creating some anxiety among colleagues who would be affected.

Yesterday Biden said he will, in effect, get the best of both worlds. He will become chairman of Foreign Relations, reflecting his interest in world affairs, while chairing a Judiciary panel that will be reestablished by Leahy to oversee the high-profile domestic issues of crime and drugs.

"Since President Bush took office, I have indicated on many occasions my deep concern about a couple of major issues," Biden said. "Chief among them are the effectiveness of our criminal justice system, the future of the Supreme Court and the administration's plans for a national missile defense."

As chairman of the Foreign Relations Committee, Biden said yesterday, he will give first priority to scrutinizing Bush's missile defense plans.

Biden has been mentioned as a possible contender for the Democratic presidential nomination in 2004, and his Judiciary position could be more helpful politically than the more prestigious post of head of the Foreign Relations panel. His announcement yesterday seemed to reflect that. He was surrounded at his news conference by law enforcement officers, and his statement focused on the Judiciary job, relegating the Foreign Relations panel to the third paragraph.

But Biden's decision to forgo the chairmanship of the Judiciary Committee came as a relief to Leahy and Harkin. If Biden had decided to take Judiciary, Leahy would likely have exercised his seniority on Agriculture, bumping Harkin in the process. Sen. Paul S. Sarbanes (D-Md.) could then have chosen either Foreign Relations or the Banking, Housing and Urban Affairs Committee. Sarbanes will now be chairman of the banking panel.

Colleagues were relieved by Sen. Joseph R. Biden Jr.'s choice.