The Tea Party movement scored another victory on Tuesday, helping to propel a dissident Republican, Christine O’Donnell, to an upset win over Representative Michael N. Castle in the race for the United States Senate nomination in Delaware.

Mr. Castle, a moderate who served two terms as governor and had been reliably winning elections for the last four decades, became the latest establishment Republican casualty. Republican leaders, who had actively opposed Ms. O’Donnell, said the outcome complicated the party’s chances of winning control of the Senate.

With all precincts reporting, Ms. O’Donnell won 53 percent of the vote to Mr. Castle’s 47 percent. The primary drew 57,000 voters, a small slice of the overall electorate.

Ms. O’Donnell, a former abstinence counselor who had failed in previous attempts to win office in Delaware, won the endorsement of Sarah Palin, Senator Jim DeMint of South Carolina and other leaders of the party’s conservative wing.