His decision makes keeping the seat in Democrats' hands risky, The Washington Post reported. With Biden bowing out, Republican Rep. Mike Castle is considered a favorite for the open seat.
"I have a duty to fulfill as Attorney General -- and the immediate need to focus on a case of great consequence," Biden wrote in an e-mail sent to supporters. "And that is what I must do. Therefore I cannot and will not run for the United States Senate in 2010. I will run for reelection as Attorney General."
His office is conducting an ongoing investigation into an alleged pedophile pediatrician.
With Biden out, Democrats likely will turn to Newcastle County Executive Chris Coons as their preferred candidate, although there are indications that former Delaware Lt. Gov. John Carney will move from the House race to the Senate contest, the Post reported.