As with the open Illinois Senate seat once vacated by Obama, Democrats have created a potential problem for themselves in Delaware, where Vice President Joe Biden vacated his longtime Senate seat to be the No. 2 in the White House. Biden's son Beau, who has been serving in Iraq with the Army National Guard, is considering a run for the seat, but he will face stiff competition from Representative Mike Castle, a popular former two-term governor and the longest-serving Congressman in the state's history. Castle is a rare species in the House these days: a Northeast moderate Republican who has broken with his party on social, environmental and budget issues — which makes him an even bigger threat to the Dems.