With polls showing significant GOP momentum this fall, Republicans in recent weeks began to believe they had a real chance of retaking control of the Senate in November. But a major primary upset at the hands of a tea party insurgent on Tuesday may have put the Senate GOP's dreams of a majority at serious risk. In the biggest electoral surprise of the night, conservative activist Christine O'Donnell defeated longtime GOP Rep. Mike Castle in Delaware's Republican Senate primary. Castle, a moderate who once served as the state's governor, had been so favored to win in November that his decision to run had reportedly influenced Democrat Beau Biden, son of Vice President Joe Biden, to abandon plans to seek his father's old seat. But with O'Donnell's come-from-nowhere win Tuesday night, top Republicans in Washington now see virtually no chance the GOP will be able to pick up the Delaware seat this fall. As a result, they admit their already slim chance of winning back Republican control of the Senate is likely dead. "It's hard to see a path for us," one senior Republican official, who declined to be named while discussing party strategy, told The Upshot. "Never say never, but it has become much harder for us after tonight." According to Public Policy Polling, just 31 percent of Delaware voters believe O'Donnell is "fit" to hold office. She trails Democrat opponent Chris Coons by 26 points, according to the latest PPP survey. On Tuesday night, the National Republican Senatorial Committee issued a tepid statement of congratulations to O'Donnell, but a GOP official told Fox News the party has no plans of putting money into the race. Still, O'Donnell's surprise victory was significant win for the Tea Party Express, which spent $250,000 at the last minute to boost O'Donnell's campaign. Since the first primaries in early spring, she's the eighth tea party-endorsed candidate to defeat an establishment-backed GOP contender in an election cycle that has been dominated by voters choosing change over experience. Two weeks ago, Alaska Sen. Lisa Murkowski lost her primary race to Joe Miller, who was backed by Palin and the Tea Party. Other surprise tea party wins among Senate candidates this year include Sharron Angle in Nevada and Rand Paul in Kentucky. In Florida, Marco Rubio was also endorsed by tea party activists, although he's tried to move toward the middle since winning the primary last month. The difference between O'Donnell and other tea party-backed Senate candidates is she's running in a state that traditionally elects moderates. O'Donnell, a perennial candidate who once argued against masturbation on a MTV special, is not likely to move toward the middle, as Rubio has, and she doesn't look to benefit from the same anti-incumbent wave that's driven Angle's poll numbers against Harry Reid in Nevada. That's the key reason why national Republicans are so loathe to embrace O'Donnell's candidacy. Not that she cares.