There’s a surprisingly hot primary going on in Delaware tomorrow that education policy folks should watch. Rep. Mike Castle, a moderate GOP lawmaker and one of the Big 8 lawmakers the administration is courting in its push to renew the Elementary and Secondary Education Act, is facing off against Christine O’Donnell, a marketing and media consultant for the GOP nomination for U.S. Senate.

Just a few months ago, Castle was expected to win this race (and the general election) without breaking much of a sweat. After all, Castle has held statewide office for years, first as governor and then in the U.S. House of Representatives as Delaware’s sole congressman.

But that was before a series of surprise upsets by candidates backed by local teaparty activists, including, most recently the victory of attorney Joe Miller over Sen. Lisa Murkowski in Alaska. Like O’Donnell, Miller had the backing of former Alaska Gov. and veep nominee Sarah Palin. This recent poll shows O’Donnell ahead of Castle, although some Castle supporters have expressed skepticism about those numbers.

What does this mean for education? Well, O’Donnell attacks Castle for being anti-school choice on her web site (he’s got a mixed record on the issue, according to this non-partisan cite, and my own experience covering Capitol Hill.) And many tea party-backed candidates have called for eliminating the U.S. Department of Education, or are, at the very least, extremely skeptical of a major federal role in schools.

Would O’Donnell actually win the seat if she got the nod? Jay Cost, who blogs for the Weekly Standard, a well-known conservative journal, doesn’t seem to think so and is urging its First State readers to vote for Castle.

What will it mean for education if Castle doesn’t serve in the Senate? Well, it’s tough to say. If he doesn’t get the nod, the odds improve substantially for Chris Coons, the New Castle County executive and likely Democratic nominee. A Coons’ win would help Democrats hold onto the Senate, and Coons is on the advisory counsel of the Rodel Foundation, an education redesign group that helped develop Delaware’s winning Race to the Top application. Check out his education views here.

But, in the House, Castle has been very interested in K-12 policy, and has helped the two parties reach consensus on education. (Check out an NCLB bill he released in 2008 here.)

In fact, Secretary Duncan gave him credit for helping to inform the administration’s ESEA blueprint, released in March. Without Castle, there would be one less moderate lawmaker that Duncan could turn for help in putting a bipartisan stamp on an eventual ESEA bill.