FAR-RIGHT HORDES HOPE TO BRING DOWN CASTLE…. Republicans will need a net gain of 10 Senate seats if they hope to control the upper chamber next year, and by most assessments, it’s a tall order. Some pick-ups, however, almost look like sure-things.

In Delaware, for example, Rep. Mike Castle (R) is one of this year’s safest bets, especially in open-seat races. This is a relatively “blue” state, but Castle is a popular representative and former governor, and has a reputation for being a GOP moderate. As a result, when Dems draw up lists of key competitive contests, Delaware doesn’t make the cut.

But Castle has a primary challenger, and the Republican base has no tolerance for those who fail to toe the far-right line. Suddenly, there’s a “possibility” that Castle “could be the next victim of the purity purge inside the GOP tent.”

Christine O’Donnell has, by and large, campaigned outside the media and political spotlight so far this election. But on Monday her efforts to take out Castle in the mid-September primary got a major boost when the Tea Party Express, which spent roughly $600,000 on Alaska Republican Joe Miller’s challenge to Sen. Lisa Murkowski, pledged to do the same on her behalf.

The announcement was just the latest in a wave of Tea party momentum to build around O’Donnell’s candidacy. The right-wing blogosphere has, likewise, either trumpeted or expressed intrigue in her campaign, disturbed, primarily, by Castle’s moderate voting record. O’Donnell herself has pushed the meme, going so far as to pursue the endorsement of Sen. Jim DeMint (R-S.C.) — the Tea Party validator in the U.S. Senate — and to include a picture of herself alongside the poster boy of Tea Party-ism: Florida Republican candidate Marco Rubio. This past weekend, in fact, O’Donnell shot footage at Glenn Beck’s Lincoln Memorial rally for future use in her campaign ads.

Castle clearly remains the favorite, but it wasn’t too long ago that Lisa Murkowski was expected to cruise past Joe Miller in Alaska, and Charlie Crist was once the prohibitive favorite over Marco Rubio in Florida.

In other words, in this environment, strange things happen. And with the Tea Party Express dumping $600,000 into Delaware to push O’Donnell over Castle, Republican leaders are getting nervous.

How nervous? Enough for party leaders to start letting political reporters know yesterday that O’Donnell “owes back taxes, had her home foreclosed on, and never received a diploma because she didn’t pay her tuition.”

The primary is two weeks from today, with the winner taking on New Castle County Executive Chris Coons (D) in November. Markos’ assessment was exactly right — if Castle wins the primary, it’s a likely GOP pickup; if O’Donnell wins the primary, Dems have reason for optimism.

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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.