[cnn-photo-caption image= http://i2.cdn.turner.com/cnn/2010/images/07/27/art.tpebus0727.gi.jpg caption="The Tea Party Express announced Tuesday that they are endorsing Christine O'Donnell, who is battling Rep. Mike Castle for the GOP Senate nomination in Delaware."]
(CNN) - The Tea Party group that helped Sharron Angle win Nevada's Republican Senate primary, but that was also kicked out of a national Tea Party federation, is jumping into the Delaware GOP Senate battle.
The Tea Party Express announced Tuesday that they are endorsing Christine O'Donnell, who is battling Rep. Mike Castle for the GOP Senate nomination.
"Christine O'Donnell has established a reputation as a strong voice for conservative constitutionalist principles consistent with the ideals of the tea party movement," said Tea Party Express Chairman Amy Kremer.
The group also criticized Castle, a popular nine term congressman and former two term governor who's considered much more moderate than O'Donnell, a conservative commentator and marketing consultant who ran unsuccessfully for the Senate in 2006 and 2008.
"We long ago announced our intention to hold Mike Castle accountable for his failed record in Congress, and now we have an excellent shot to make sure he is defeated by a solid conservative candidate," said Tea Party Express Coordinator Joe Wierzbicki.
The early morning email release announcing the endorsement surprised the O'Donnell campaign, which says they didn't get a heads up from Tea Party Express. But O'Donnell says she's honored to receive the endorsement.
"We share the same goal of a restoration of the Constitutional principles of limited government, lower taxes, peace through strength and a pro-growth economic environment that opens wide the gates to the American Dream for all Americans," said O'Donnell in a statement. "The Tea Party Express represents the mainstream sentiment that it's time to roll back the radical, Big-Government agenda issuing out of Washington, and I am proud to stand with them on behalf of Delaware and all freedom-loving Americans."
Tea Party Express is one of the most recognized national Tea Party organizations. Until two weeks ago the group was best known for its three high-profile bus caravans and rallies.
The Tea Party Express has also become a major player in Republican politics, spending hundreds of thousands of dollars on ads for Republican Senate candidate Scott Brown in Massachusetts who upset Democrat Martha Coakley to fill the late Sen. Ted Kennedy's seat.
More recently, the organization helped the little-known Angle win the Republican primary in Nevada to face Senate Majority Leader Harry Reid, endorsing her and spending about a half-million dollars on ads for her. The organization says it plans to spend more to help her get elected.
The group also targeted incumbent Sen. Bob Bennett in Utah for his vote on the Troubled Assets Recovery Program and helped defeat his bid for a fourth term. It then backed underdog Mike Lee's successful primary campaign and Lee now appears to be a shoo-in to win the general election in November and join the Senate next year. In Alaska, it's now backing conservative Joe Miller, who is challenging Republican Sen. Lisa Murkowski in the GOP primary.
But in the past two weeks, the National Tea Party Federation, which seeks to represent the Tea Party political movement around the country, expelled the Tea Party Express because of an inflammatory blog post one of its leaders, Mark Williams, wrote responding to criticism from the NAACP.
Williams' blog post satirized a fictional letter from what he called "Colored People" to President Abraham Lincoln. Williams has since left the organization.
Tea Party Express won't say at this point if they'll run ads for O'Donnell, who is considered a long-shot against Castle in the September 14 primary.
The winner of November's general election will fill out the remaining four years of Vice President Joe Biden's final term in the Senate. Biden stepped down from the Senate after his election in November 2008 as vice president. Former Biden aide Ted Kaufman was named as an interim replacement. Kaufman is not seeking a full term.
After much speculation that he would run for his father's old seat, Delaware Attorney General Beau Biden announced in late January that he would instead run for re-election as his state's top lawyer. Nine days after the younger Biden's announcement, New Castle County Executive Chris Coons officially announced his candidacy for the Democratic nomination.
Political handicappers consider the Delaware seat a prime pick-up possibility for the GOP.
Follow Paul Steinhauser on Twitter: @psteinhausercnn