GOP SENATE PRIMARY HEATS UP IN DELAWARE…. Of all the Republican Senate pick-up opportunities, Delaware was supposed to be one of the easy ones. The seat once held by Vice President Biden is open this year, and Rep. Mike Castle (R), a relatively moderate former governor, was largely seen as a shoo-in.

But as we’ve seen repeatedly this year, GOP primaries can shake things up in unexpected ways. Castle’s swearing in has been rudely interrupted by right-wing activist Christine O’Donnell. The challenger has clearly generated genuine fear within the Castle camp — his campaign started spending money a couple of weeks ago that it never expected to be necessary — and a new PPP poll shows O’Donnell inching ahead.

It looks like there’s a real possibility of a major upset in the Delaware Senate primary on Tuesday night, with insurgent conservative Christine O’Donnell leading longtime Congressman and Governor Mike Castle 47-44.

It’s hard to compare these results with other recent polling — since no one expected this to be competitive, there just aren’t many other polls — but given the wild-eyed panic within the Republican establishment, it’s likely internal surveys are offering similar data.

Also note, it’s not just the polls giving O’Donnell a sense of momentum. Last week, former half-term Gov. Sarah Palin (R) threw her support to the far-right challenger, and over the weekend, primary voters started receiving robo-calls from Palin. Likewise, both the National Rifle Association and right-wing Sen. Jim DeMint (R-S.C.) endorsed O’Donnell in recent days.

Hoping to rescue the one-time favorite, the Republican establishment is furiously going after the challenger. Last week, for the example, the Republican Party of Delaware filed a formal complaint with the Federal Election Commission, accusing O’Donnell of illegally collaborating with the Tea Party Express. (State parties don’t usually file complaints with the feds against one of their own U.S. Senate candidates.) The party is also dishing dirt on O’Donnell to Republican media outlets.

And just as an added twist, also note that Castle could still pull a Lieberman and run as a third-party candidate if he loses the primary, though he’s been reluctant to talk about the option.

If I had to put money on the race, I’d say Castle still manages to eke out a win when primary voters head to the polls tomorrow. But I thought Sen. Lisa Murkowski would win in Alaska, too, and it’s proving to be one of those years for the Republicans’ far-right base.

If you’re thinking that New Castle County Executive Chris Coons, the presumptive Democratic nominee, is enjoying all of this, we’re on the same page.

Steve Benen

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.