Long-shot U.S. Senate candidate Christine O'Donnell of Delaware rode tea-party momentum Tuesday to the most unexpected upset of a tumultuous political year, soundly defeating Republican Party fixture Mike Castle in a bruising primary.

The results showed again that conservative voters, many of them angry at federal spending by both parties and energized by the tea-party movement, have become mobilized even in one of the nation's most Democratic-leaning states.

Anti-establishment...