O'Donnell also faces a numbers battle. She garnered less than 30,000 votes in a contest in which only Republicans could cast ballots. About 293,000 Democrats are registered in Delaware, compared with about 183,000 Republicans.
Her supporters remained undaunted.
"If she can beat Castle, she can beat Coons," said Judy Mangini, 50, a real estate agent from Lewes who worked on O'Donnell's campaign.
GOP support for O'Donnell is doubtful not only because of questions about her appeal to the general electorate and her finances, but because of the harsh words leveled at Castle during the primary.
Besides casting him as a liberal, O'Donnell, 41, and her supporters suggested that the 71-year-old Castle is so frail that he might die before finishing his Senate term, that he might switch parties and become a Democrat, and that he was cheating on his wife with a man.
"This has not been at all pleasant, this particular campaign," Castle said.