There could be an upset in the Delaware GOP Senate primary, according to a new poll out Sunday.

Republican Christine O’Donnell has a slim lead over Rep. Mike Castle (R-Del.) with only 24 hours before voting starts. O'Donnell was ahead 47-44 in the survey of 668 likely Republican primary voters on Sept. 11-12. That lead, however, falls within the poll's margin of error.

Castle led 69-21 among centrist voters, which is the group his campaign is focusing on turning out. O'Donnell led 62-31 with conservative voters.

The PPP poll from New Hampshire didn't suggest an upset was as likely there. Establishment favorite Kelly Ayotte (R) led the GOP Senate primary field with 37 percent of the vote. Former gubernatorial candidate Ovide Lamontagne had 30 percent and businessman Bill Binnie was third with 13 percent.