Partisan Democrats are delighted about Christine O’Donnell’s Republican primary victory over Rep. Mike Castle in the race for the open Delaware Senate seat.

I’m despondent.

From the Democratic point of view, the defeat of the moderate, well-known Castle turns what had looked to be a lost cause into a likely win. Keeping the seat in Democratic hands could be the margin of control in the Senate. So the folks who focus on electing Democrats and keeping a Democratic majority can’t be blamed for breaking out the champagne over O’Donnell’s win.

Not me, for two reasons.

First, I had thought the silver lining of this election year might be to produce a Senate with a more robust cadre of moderate Republicans. That caucus has pretty much dwindled to the two senators from Maine, with very occasional company from colleagues such as Massachusetts Sen. Scott Brown and departing Ohio Sen. George Voinovich. It’s awfully hard for a caucus of two to break with the party.

Peer pressure isn’t just a phenomenon of middle school. It’s alive and well in the U.S. Senate, and Minority Leader Mitch McConnell, R-Ky., has done a good job of keeping party discipline. A larger number of moderates among his herd of cats might make that more difficult and enhance the prospects for bipartisan legislating. There is strength in numbers, and you could imagine a bolstered group of (at least relative) moderates made up of the likes of Castle, Carly Fiorina (Calif.), Mark Kirk (Ill.) or Dino Rossi (Wash.)

Now, it’s as plausible to envision a bolstered Jim DeMint caucus, following the disturbingly powerful junior senator from South Carolina: Sharron Angle (Nev.), Rand Paul (Ky.), Ken Buck (Colo.) — plus the two other incumbent-slayers of the primary season, Mike Lee in Utah and Joe Miller in Alaska. Scary.

But not as scary as reason No. 2: the ripple effect of victories such as O’Donnell’s on other Republican lawmakers. Republican members of Congress look at races such as those in Utah, Alaska and now Delaware and think: There but for the grace of the tea party go I. They will be that much more watchful of protecting their right flank against a primary challenge. They will be that much less likely to take a political risk in the direction of bipartisanship. In this sense, it matters less whether O’Donnell will win the general election — that doesn’t seem likely — than that she won the primary.

The Delaware result might be good news for both tea partyers and Democrats. It is not good news for the cause of good government.

Ruth Marcus’ e-mail address is marcusr(at symbol)

© 2010, Washington Post Writers Group

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