Paul Donnelly, 53, lives in Smyrna.
Donnelly said he was inspired to run because Castle is backing a bill that could fundamentally change the federal Head Start program, which provides education and other services to more than 900,000 poor kids at 1,670 centers around the country.
One part of the legislation would allow the federal government to set up eight pilot programs around the country in which states would receive block grants to run their own Head Start programs. The bill has passed the House and is in a Senate committee.
"This is something that works very well," Donnelly said of Head Start. "One of the strengths of the program is the work we do with the families and, if this shifts out of the Department of Heath and Human Services, we could lose that."
He claimed the changes are part of the Bush administration's social services agenda.
"People have the impression that Mike Castle is a nice, middle-of-the-road guy, but then he carries the administration's water on something like this," Donnelly said.
Castle spokeswoman Elizabeth Wenk said the congressman welcomed Donnelly to the race.
Donnelly said he would take a leave of absence from his job during the campaign to avoid the appearance of a conflict of interest.
He said he has filed candidacy papers with state and federal election officials.
Nicole Majeski, the state Democratic Party's executive director, said she was pleased Donnelly was joining the ticket.
At a Democratic Party district meeting at Rep. Bruce C. Ennis' Smyrna home, Donnelly said that the party needed to find someone to take on Castle, 65, who is seeking a seventh term.
"When I made the suggestion, everyone shook their heads and said Mike Castle's unbeatable," Donnelly said. "But afterward, Bruce took me aside and asked if I'd be interested in running."
Castle was re-elected in 2002, defeating Democratic Mike Miller for the second time and taking 72 percent of the vote.
Donnelly also said Castle needs to be held accountable for his support of the war with Iraq. Donnelly's son is an Army medic who recently left the service after a tour of duty in Iraq.
"I'm glad he's back, but there are thousands of other families with loved ones over there," Donnelly said. He said that several members of Congress have said that if they knew what they know now — that no weapons of mass destruction have been found in Iraq — they would not have voted to support the war.
"I'd like to hear Congressman Castle say that he made a mistake, but I doubt if he will," Donnelly said.