Christine O'Donnell

WASHINGTON — The insurgent "tea party" movement vanquished the Republican Party establishment Tuesday as an underdog candidate for U.S. Senate in Delaware won a primary that had become national proxy war for the heart of the GOP.

Conservative activist Christine O'Donnell, a perennial candidate with no experience in elected office, defeated longtime Rep. Michael Castle in a dramatic finale to a primary season dominated by the small-government movement's revolt against more-moderate elements of the GOP.

"Don't ever underestimate the power of we the people," O'Donnell, 41, told a group of jubilant supporters Tuesday night. "We the people will have our voice heard in Washington, D.C. once again."

While a victory for grass-roots conservatives, O'Donnell's win was a significant setback in Republicans' quest for control of the Senate from Democrats. Castle, the longest-serving congressman in the state's history, had been considered a shoo-in for the seat vacated by Joe Biden, but O'Donnell is likely to have a harder time winning over independent and moderate voters in her race against New Castle County Executive Christopher Coons, the Democratic nominee.

"You have a very inexperienced candidate in O'Donnell who has flaws that are even being pointed out by Republicans, against Chris Coons, who is a very good candidate," said Nathan Gonzales, an analyst for the nonpartisan Rothenberg Political Report. Gonzales called the result "a game-changer."

The conflict between the tea party and the political establishment has defined the primary season, which began to wrap up Tuesday with contests in seven states and the District of Columbia. Hawaii holds its primary on Saturday.

In New York, Carl Paladino, a millionaire developer aligned with the tea party, knocked out former Rep. Rick Lazio in the race for the Republican nomination for governor. State Attorney General Andrew Cuomo looms as the Democratic nominee for the open governorship.

In New Hampshire, another establishment candidate struggled to fend off a challenge from the right. As vote counting went late into the night, Ovide Lamontagne, an attorney who had support among the state's traditional conservative base, took a surprising early lead over former state Attorney General Kelly Ayotte.