WASHINGTON – One week after helping engineer a surprise in Alaska, the Tea Party Express announced a six-figure commitment Monday to back Christine O'Donnell and her challenge to Delaware Rep. Mike Castle for the state's Republican senatorial nomination.

A spokesman, Levi Russell, said the organization hopes to begin airing radio and television ads by the end of this week or early next week for the Sept. 14 primary, and put the anticipated cost at about $250,000.

The Tea Party Express was one of several political forces that coalesced behind Alaska challenger Joe Miller in the final weeks of a primary race against Sen. Lisa Murkowski, spending several hundred thousand to help him. Nearly a week after the polls closed, Miller held a lead of about 1,700 votes, with several thousand absentee and challenged ballots yet to be counted.

In recent days, several Republican officials have said that Murkowski was so confident of victory that she turned aside advice from strategists to unleash an attack against Miller. They spoke on condition of anonymity, saying they were not authorized to discuss strategic discussions inside the campaign.

A similar choice may soon confront Castle, a leading moderate among House Republicans, who leads O'Donnell in private polls and is counted a favorite to win a seat long in Democratic hands.

However Castle responds, GOP officials moved quickly to try and discredit O'Donnell, citing a published report from last spring that the Internal Revenue Service had placed a lien against her for nonpayment of taxes and that her home had been subject to foreclosure proceedings.

In a rebuttal, O'Donnell's campaign forwarded a mid-August blog posting from her website that said the IRS had "admitted the (lien) letter was a mistake and chalked it up to a 'computer error.'" Later, a statement said the attacks were "obscene, insane borderline slander...

O'Donnell's campaign website lists her as a "nationally recognized political commentator and marketing consultant" on Fox News and elsewhere, and also a frequent radio talk show guest host in Delaware.

Castle is a former two-term governor who has been elected to the House for nine terms, generally winning by overwhelming margins in a state that has grown more Democratic in recent years.

His campaign manager, Mike Quaranta, said the Tea Party Express had announced several weeks ago it would become involved in the race. "We have planned for that action and we have a campaign plan and we're just going to continue to work it," he said.