Biden's decision is a big blow to Democrats, as they had said for months that they expected him to run, even as he hesitated upon a return from a deployment in Iraq. It is the latest bad news for Democrats in a cycle with a growing number retirements and candidates bowing out.
It's not clear where Democrats go from here. Castle, a former governor, remains very popular in the state, and should be a strong favorite against anyone not named Biden.
Vice President Joe Biden was quoted in a newspaper column Sunday saying he didn't think his son would run. Biden's office soon disputed the report, and it was retracted.