WHEN A CASTLE GETS TORN DOWN IN DELAWARE…. Prominent far-right blogger Erick Erickson noted recently that a vote for a right-wing extremist candidate in Delaware’s GOP Senate primary would likely hurt his party, but he just didn’t care. “I’d rather see the Democrat get elected than see Mike Castle get elected,” he said. “Seriously, I know many of you disagree with me, but if the majority depends on Mike Castle, to hell with the majority.”

The Republican Party desperately hoped primary voters in Delaware disagreed. Indeed, this was supposed to be one of the year’s easiest pick-up opportunities — Rep. Mike Castle (R), a relatively moderate former governor, is a giant of Delaware politics, and Democrats had no real expectations of beating him in November. The party reveled at the symbolic significance of winning Vice President Biden’s old seat just two years after the presidential race.

And now that excitement has vanished. A race Republicans expected to win fairly easily is now a contest Democrats expect to win fairly easily.

Christine O’Donnell’s surprise victory in the Delaware Senate GOP primary Tuesday left Republicans in conflict, senior party officials openly fretting that the Senate is now out of reach and Democrats overjoyed that the opposition has handed them a late and desperately needed chance to reframe the national argument about the 2010 elections.

Aside from the political implications of the upset, the outcome prompted a round of deep Republican soul-searching about what it said about their party when a political pillar in Delaware like Rep. Mike Castle, a respected lawmaker who was considered a shoo-in for the Senate seat, could not even come within six points of defeating the controversial and still largely unknown O’Donnell.

As the dust settled, and with all the precincts reporting, an apparent crazy person had won 53% of the vote to Castle’s 47%. Only about 57,000 voters participated in the primary election.

This isn’t the first time hysterical Tea Party activists bucked the party’s leadership to nominate an extremist — see recent results in Alaska, Nevada, Kentucky, Colorado, and New York — but it was supposed to be different in Delaware. The GOP establishment and its preferred candidate saw this coming weeks ago, and acted quickly to prevent a disaster, attacking O’Donnell, dishing dirt to reporters, and even filing an FEC complaint against her.

It didn’t matter. When inmates run asylums, it never does.

To be sure, it’s a strange year, and simply writing off O’Donnell’s chances in November would be a mistake. Dems have every reason to be optimistic, but they can’t take anything for granted.

That said, O’Donnell instantly joins the Crazy Caucus and may in fact be in the running for the nuttiest of them all. She has, after all, equated masturbation with adultery as part of her bizarre religious crusade. The chairman of the Delaware Republican Party characterized his party’s U.S. Senate nominee as “a perennial candidate unworthy of being elected dog catcher.” Even Dick Armey’s FreedomWorks, which rallies behind right-wing fringe candidates, wanted nothing to do with her.

By last night, the National Republican Senatorial Committee had sent word that it will not make any investments in O’Donnell’s general-election campaign, and Karl Rove declared the race a lost cause for the GOP.

Given the political trajectory of the last several months, it’s hard to believe Democrats could be this lucky.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.