And with recent independent polls showing Castle as having the greater chance of prevailing over Democratic county executive Chris Coons in the general election, Republican party leaders are eager to do whatever it takes to get Castle on the ballot.
"She is not a viable candidate for any office in the State of Delaware," state GOP party chairman Tom Ross told AP of O'Donnell. "She could not be elected dog catcher."
Former Republican Sen. Norm Coleman, who now runs a conservative political advocacy group, said, "Mike Castle is going to win his race in Delaware. ... It's just a personal observation from someone who hopefully knows a little bit about what is going on out there.
"But in this day and age, right now, there are a lot of surprises."
The potential for a surprise in Delaware has political strategists and party operatives on both sides watching closely. It's also keeping O'Donnell optimistic.
"My opponent has a sense of entitlement and assumes Delaware voters are just going to hand him a U.S. Senate seat," she said last week, "but I prefer to hand him a pink slip."