(CNN)– With one day to go until the primary, an ugly GOP Senate nomination battle in Delaware is getting even uglier.
The Tea Party Express, the California based national Tea Party organization that's backing conservative candidate Christine O'Donnell, is demanding that Delaware Republican Party Chairman Tom Ross resign.
Ross is supporting former governor and nine-term state-wide Rep. Mike Castle in the contest. In May, at the state GOP convention, the moderate congressman and former two-term governor defeated O'Donnell, a conservative commentator and marketing consultant who ran unsuccessfully for the Senate in 2006 and 2008.
Tea Party Express endorsed O'Donnell earlier this summer and in the past two weeks went up with ads attacking Castle as too moderate. The group says they'll spend at least $250,000 on behalf of O'Donnell. A similar last minute blitz by the Tea Party Express last month was credited in helping conservative candidate Joe Miller top Sen. Lisa Murkowski in Alaska's Republican primary.
Ross has become very vocal in the past two weeks in criticizing O'Donnell and the Tea Party Express's involvement in the race.
Sunday night the Tea Party Express laid out an ultimatum.
"Tom Ross must resign within the next 24 hours or face immediate termination," declared Amy Kremer, Chairman of the Tea Party Express. "As Mike Castle's chief advocate, Mr. Ross has trashed Republican candidates, beliefs and principles, and shown a complete lack of character or integrity. He is a walking disaster who has brought irreparable harm to the Republican Party."
In an email release, the Tea Party Express said they were calling for the resignation in conjunction with a coalition of Delaware Republicans and conservative activists.
In a statement to CNN, the Delaware Republican Party fired back.
"As party chairman, Tom Ross is supporting the candidates that were elected by grassroots Delaware Republicans at the convention in May. A California political consultant who's never lived a day of her life in Delaware and is currently under investigation by the FEC for illegally coordinating with the O'Donnell campaign is the last person who should be calling for anyone's resignation," said Delaware GOP Communications Director Tom Doheny.
The investigation Doheney mentioned refers to a formal complaint the Republican Party of Delaware filed with the Federal Election Commission, accusing one of its own Senate candidates of illegally collaborating with the Tea Party Express. The Tea Party Express denies wrongdoing.
The series of TV and radio ads by the Tea Party Express attack Castle as a liberal candidate who "just keeps supporting the failed policies of Nancy Pelosi and Harry Reid."
The Castle campaign has also fought back.
"Out-of-state interest groups have threatened to spend half a million dollars to fund the disgusting tactics being used by the O'Donnell campaign to make accusations," said Castle campaign manager Mike Quaranta.
Meanwhile, the Castle campaign has launched therealchristine.com, a site devoted to aggregating negative news about O'Donnell. O'Donnell has faced criticism over her personal finance issues and leftover debt from previous, unsuccessful bids for a Delaware Senate seat. She has also been accused of misstating the results of her run against Joe Biden.
The winner of Tuesday's primary will face off in November against New Castle County Executive Chris Coons, the presumptive Democratic nominee, in the battle to fill the final four years of Vice President Joe Biden's final term. Biden stepped down from the Senate after his election in November 2008 as vice president. Former Biden aide Ted Kaufman was named as an interim replacement. Kaufman is not running for a full term.
Follow Paul Steinhauser on Twitter: @PsteinhauserCNN
CNN's Alison Harding contributed to this report