Christine O'Donnell can't win a two way race. The Delaware electorate will be nearly 50% Democratic. Massachusetts is only 37% Democratic. California will be 40%-42% Democratic. While a Republican candidate can win in blue states with independent support, they can't win in states where the electorate is around 50% Democratic without Democratic support. O'Donnell won't get that. She'd need to pretty much win all the Republican and independent votes to win. She's going to lose Republican votes from Castle supporters. Independents are less enamored with her.
Mike Castle has strong Democratic support. In a head to head match-up with Chris Coons, Castle polled at 30% Democratic support, compared to 13% for O'Donnell. Castle is likely to appeal stronger to Coons voters than O'Donnell voters. With the recent primary being so bitter, there's no guarantee Castle voters will back her. Her core conservative supporters won't abandon her. While she probably would top out in the low 40's in a 2-way race, she could get in the mid 30's in a three way contest.
Mike Castle can win. I know some Republicans would rather have Coons than Castle, but this isn't a Charlie Crist or Lisa Murkowski candidacy. Castle has won four gazillion statewide elections. Crist won 3. Murkowski 1. Castle has the advantage of an ugly primary that's recent. Crist Republicans didn't work up animosity against Rubio, so they had time to decide they wanted to vote Republican. This bitter election is so recent and bitter that Castle could hold many of his supporters. Unlike Crist and Murkowski he's done well with Democrats and won independents in the past. He could suck enough votes from either of them for a win. I'm sure conservatives imagine the worst from him, but there's no indication he'd do anything different than he's always done and caucus with the Republicans.