Updated: 7:41 p.m.

Former Alaska Gov. Sarah Palin waded into Delaware’s Republican Senate primary on Thursday by endorsing Christine O’Donnell, the tea party-backed candidate who is giving headaches to the state and national GOP establishment.

O’Donnell disclosed the endorsement in a Twitter message at around 4 p.m. “Just got Gov. Palin’s endorsement! Thank you for your prayers!” she wrote.

Delaware voters will head to the polls Tuesday to decide between O’Donnell, the outspoken conservative, and longtime Rep. Mike Castle, a moderate former governor. Republicans in Washington and Delaware alike have rallied against O’Donnell in recent days, criticizing her “vulgar attacks” against Castle and calling her a liar.

Provided he wins the nomination, Castle would be viewed as the favorite to win the general election against county executive Chris Coons, the Democratic nominee. But if O’Donnell pulls off an upset, Democrats would no doubt have the upper hand in the race for the seat formerly held by Vice President Joseph Biden.

The Tea Party Express sent a team of 10 leaders to Delaware earlier in the week to back O’Donnell.

“All of our efforts and resources are focused on Delaware. We’re not going to get into any other races before next week,” Tea Party Express Chairman Amy Kremer said in a phone interview from Delaware, where her organization will host several events and get-out-the-vote efforts for O’Donnell through the primary. “When we get behind a candidate, we go all in.”

The Castle campaign released a statement in response to the endorsement that said the primary will be decided by “grassroots voters here in Delaware, not out of state interest groups who are working to control the outcome.”

“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,” Castle campaign spokeswoman Kate Dickens said.