The former two-term governor will make the announcement at a news conference later Tuesday, according to a person close to Castle and familiar with his decision but who requested anonymity because it was not yet official.
The 70-year-old Castle's decision could pit him against Biden's son, Delaware attorney general Beau Biden, who is weighing a bid for the Democratic nomination.
The Senate seat is now held by longtime Biden aide and confidante Ted Kaufman, who was appointed to fill the seat by former Democratic Gov. Ruth Ann Minner and has said repeatedly that he will not run in 2010.
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.
DOVER, Del. (AP) _ Delaware's longest-serving congressman, Republican Michael Castle, plans to settle speculation and announce if he'll run for Democratic Vice President Joe Biden's former U.S. Senate seat.
The nine-term representative and former two-term governor scheduled a news conference about his future political plans for Tuesday afternoon in Wilmington.
The 70-year-old Castle has said he was leaning against seeking a 10th term in the House, but instead would weigh whether to run for the Senate or retire. He's never lost a political race since being elected to the state House in 1966.
If Castle does run for the Senate, he could find himself in a contest against Biden's son, Delaware Attorney General Beau Biden, who has not yet announced his plans.