The moderate Castle has been co-sponsor with Representative Diana DeGette (D–CO) of a bill to expand federally funded research on hESCs that passed Congress twice but was vetoed by then-President George W. Bush. Although stem cells weren't a major issue in the Delaware race, O'Donnell, an abortion opponent, did criticize Castle for his support of the research. County Executive Chris Coons, who easily won the Democratic nomination for Senate, supports Castle's position on stem cells.
The prospects for stem cell legislation are uncertain.