HOUSE INCUMBENT TAKEN DOWN IN DEM PRIMARY…. As a rule, House Democratic incumbents fare pretty well in primary campaigns when challenged from the right. There are, however, exceptions.

Rep. Alan B. Mollohan (D-W.Va.) lost his bid for a 15th term Tuesday in a primary defeat that further affirms the anti-incumbent sentiment coursing through the country.

He is the first House member to lose a reelection bid in the 2010 campaign, and his defeat comes days after Sen. Robert F. Bennett (R-Utah) was knocked off the November ballot in that state’s convention process.

Mollohan hadn’t faced a serious primary fight in more than a decade and was seen in some circles as unbeatable, given that the state’s 1st Congressional District seat had been in his family since 1968. (His father held it for seven terms before he won it.)

But state Sen. Mike Oliverio ran hard against Mollohan’s entrenched-incumbent status and made much of the lingering whiff of ethics problems that dogged the congressman for years.

It wasn’t especially close — with nearly all the precincts reporting, Oliverio beat Mollohan, 56% to 44%.

The parallels with Bennett’s recent defeat in Utah are a little misleading. Bennett lost because the Republican base decided he wasn’t right-wing enough. In West Virginia, Mollohan wasn’t “purged” by the Democratic base for being a moderate; the incumbent’s challenger was far more conservative than he was.

So, what happened here? Most of the media analysis will focus on the lousy climate for incumbents, and that was certainly part of the equation in WV1. But the variables paint a more complex picture. Mollohan faced a lengthy ethics investigation, and while he never faced formal charges, the probe made it easy for Oliverio to raise questions about the incumbent’s integrity.

Moreover, by all accounts, Mollohan simply didn’t take the primary threat seriously until it was too late. After 14 terms, he grew complacent. Indeed, in 2008, Mollohan didn’t even face a Republican challenger (he also ran unopposed in 2002 and 2000).

The issues didn’t help, either. Conservative opponents of abortion rights attacked Mollohan for supporting the Affordable Care Act — there are a lot of pro-life Dems in West Virginia — and while Mollohan opposed cap-and-trade, he was perceived as not having fought it aggressively enough.

As for what’s next, Republicans conceded that they hoped Mollohan would win the primary, because they saw him as easier to beat in November. In this sense, the Democrats’ odds of holding onto the seat may have actually improved slightly in light of the primary results.

But an Oliverio victory in the general election may not mean much to the party in 2011 — the conservative state lawmaker would vote with Republicans on nearly everything, and has already said he doesn’t expect to support Nancy Pelosi for Speaker. Oliverio is, in other words, practically the quintessential “Democrat in Name Only.”

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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.