Washington (CNN) - Former Alaska Gov. Sarah Palin joined the Tea Party Express Thursday in endorsing Republican Senate hopeful Christine O'Donnell, who is locked in a battle for Delaware's GOP nomination with centrist Rep. Mike Castle.
Palin, the 2008 Republican vice presidential nominee, made the endorsement in an interview on conservative commentator Sean Hannity's radio show.
O'Donnell quickly touted Palin's backing.
"Just got Gov. Palin's endorsement! Thank you for your prayers!" O'Donnell wrote in a message on Twitter.
The state's GOP establishment has thrown its weight firmly behind Castle, a former Delaware governor who has been serving in Congress since 1993.
The Castle campaign responded to Palin’s backing of O’Donnell by pointing to their own endorsements from “respected conservative Delawareans.”
"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. 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," said Castle spokesperson Kate Dickens.
The Tea Party Express on Tuesday announced its support for O'Donnell, outlining plans for an aggressive campaign effort in the days leading up to the September 14 Republican primary.
UPDATE: Late Thursday night, the Republican Party of Delaware, which has voiced support for Castle, said in a release that it has filed a complaint with the Federal Election Commission alleging that O’Donnell has knowingly accepted illegal donations from the Tea Party Express.
Read the FEC complaint here.
– CNN’s Brian Todd and Mark Preston contributed to this report.