Tea Party-backed Christine O'Donnell said Thursday a late endorsement from former Alaska Gov. Sarah Palin (R) proves that her campaign has the momentum it needs to upset Rep. Mike Castle (R-Del.) in the state's GOP Senate primary Tuesday. 

"Something really special is going on here in Delaware," O'Donnell told The Hill late Thursday. "This just increases the excitement around our campaign right now and that momentum will carry us through."

O'Donnell said she was at a local restaurant talking to voters when she received a text message alerting her of Palin's endorsement. The former Alaska governor announced her backing of O'Donnell on conservative talk-show host Sean Hannity's radio show Thursday. 

"She stood up against the establishment just like I have in Delaware and she blazed a trail for insurgent candidates," O'Donnell said. 

Palin later posted the endorsement of O'Donnell on her Facebook page. "Please support Christine O’Donnell in Delaware," Palin wrote. "She will support efforts for America’s energy security, patient-centered health care reform, cutting government waste, and letting the private sector thrive and prosper!"

The O'Donnell campaign did not know the endorsement was coming. 

The candidate said Palin's backing comes amid "growing frustration with the tactics" of the Castle campaign and the Delaware Republican Party.

The state GOP filed a complaint with the Federal Election Commission Thursday alleging illegal coordination between the Tea Party Express and the O'Donnell campaign. O'Donnell declined to comment on the complaint. 

Castle's campaign dismissed Palin's backing and touted his own endorsements Thursday. 

"Tuesday's primary will be decided by grassroots Republican voters here in Delaware, not out-of-state interest groups who are working to control the outcome," Castle spokeswoman Kate Dickens said in a statement. "Mike Castle has overwhelming support from respected conservative Delawareans, including former Gov. Pete duPont and Judge Bill Lee, who know that Castle is the true fiscal conservative and the only candidate who can win this seat for Republicans in November."