Chris Coons (D) has announced his candidacy for the Senate seat formerly held by Vice President Joseph Biden.

“I’m running for Senate because it’s time for new leaders to assume responsibility in Washington,” Coons, a county executive in Wilmington, said in a statement posted on his campaign Web site.

The party turned to Coons after the vice president’s eldest son, Delaware Attorney General Beau Biden (D), opted not to run, depriving Democrats of their strongest possible candidate against Rep. Mike Castle (R).

Democrats say they like the contrast between the 46-year-old Coons, who was re-elected in 2008 in Delaware’s most populous county, and the 70-year-old Castle, who has been a Delaware officeholder for most of the past 40 years.

“This will be a race about who is positioned to lead Delaware into the future, and given his accomplishments, Chris Coons is well-positioned as an outsider to make that argument,” Democratic Senatorial Campaign Committee Chairman Bob Menendez (N.J.) said in a statement.

The National Republican Senatorial Committee said in a statement that “we’re confident that voters will reject Chris Coons’ tax-and-spend agenda and elect Mike Castle to the U.S. Senate in November.”

Sen. Ted Kaufman (D), who was appointed in early 2009 to succeed the elder Biden, is not defending the seat.