By NEIL KING JR.

Delaware's Republican establishment dismissed Christine O'Donnell as a gadfly with a flimsy resume and shallow roots in the state.

But on Tuesday, the tea party activist beat the state's best-known Republican in her campaign for Vice President Joe Biden's old Senate seat, becoming another rising star in one of the most volatile political seasons in decades.

A freelance marketer and Philadelphia native who moved to Delaware seven years ago, Ms. O'Donnell knocked off Rep. Mike Castle, whose 44-year political career included stints in the state legislature, eight years as Delaware governor and nine terms as the state's lone congressman.

She polled 53.1% of the votes in the race, with Mr. Castle getting 46.9%. Ms. O'Donnell now will face Democrat Chris Coons in November for the seat vacated by Mr. Biden after he was elected vice president.

She pulled off the upset on a shoestring budget—running the campaign out of her small townhouse in Wilmington—and in the face of vigorous attacks from the Delaware Republican Party and the National Republican Senatorial Committee.

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