"As the Castle Campaign continues to sink in the polls, he is desperately digging up issues to distract voters from the fact that he voted with President Obama on just about every major spending bill this summer," O'Donnell wrote on her blog.
"Sad but true, his tactics have dominated a whole news cycle distracting some reporters from the real issues, like how we're going to get private sector jobs back in Delaware or why he voted for Cap and Trade."
Still, it's clear that at least part of the challenge facing O'Donnell is in getting her message out in the final days of the campaign. With only $20,000 in her war chest -- and $10,000 of debt -- she'll be unable to compete with the same intensity as Castle, who has $2.6 million in his accounts, according to the most recent Federal Election Commission filings.
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."
The winner of Tuesday's primary contest will face Democrat Chris Coons in November in a race for the seat Vice President Joe Biden occupied for 36 years.