1. You remember the Letterman bit, "Is This Anything?" Cue the hula hoop girl and the grinder girl: no question, Paul, that this is something. There are only 100 Senators, and even fewer Senate seats that are usually competitive. Given the structure of the Senate, each Senator is quite important; it's not like the House, where party control is most of the battle. And, even for party control,
2. On the other hand, while the effects certainly are likely to matter, I'd be careful about overinterpreting what exactly happened. First of all, O'Donnell prevailed by all of 3500 votes; yes, it still would have been interesting if she had just fallen short, but we wouldn't be talking about it today. Odd things happen in low-information primaries, and I'd hesitate to generalize about them. For example, Nate Silver is saying that this is "an emphatic reminder that voters write the script. The rest of us self-proclaimed political professionals – journalists and pollsters, activists and bundlers, lobbyists and party-leaders, presidents and senators — are just the stagehands." Perhaps, and of course ultimately it is the voters who make the final choices, but without further analysis we don't know the extent to which those choices were influenced by activists and party leaders. Jonathan Chait has a fun piece in which he posits that Delaware was a case of party leaders trying and failing to rein in the excesses that they have been encouraging for the last two years. Maybe; but my understanding is that talk radio hosts including Rush Limbaugh have been supporting O'Donnell, not Castle, and I'm pretty confident that a lot more Delaware Republican primary voters listen to Rush than read the Weekly Standard. I'm not saying that Chait is wrong; I just think we don't yet know exactly what happened and why.
3. Of course, Delaware wasn't an isolated case. This has truly been a fascinating primary cycle, especially on the Republican side. Again, I'd separate the effects (likely some 1-4 fewer GOP Senators taking office in January) from the explanations of what exactly has happened. Is this, as Ed Kilgore thinks, "the final victory in a Fifty Year War waged by the conservative movement for control of the Republican Party"? Maybe (and I like Kilgore's post in general; I think he's right about Tea Partyism), but I'm not confident about that. Another way to look at it is in terms not of ideological differences, but in terms of a fight within the conservative movement between purists and professionals over best tactics. Or perhaps we're seeing proxy skirmishes in the 2012 presidential nomination process (DeMint? Yup, he's a player). Or perhaps it's a handful of local situations (purist candidates defeating weak alternatives in Colorado and Nevada, a local personal feud in Alaska, a strong candidate possibly miscast as a purist in Florida, and then in Delaware a real honest-to-goodness moderate who, unlike almost all of the others, really was out of step with his party on the issues. It's also possible that these primaries really have been about voters who (presumably as a result of the economy) simply are fed up with incumbents, as some have claimed. Or maybe it's a case of internal divisions among conservative elites along some fault line we haven't yet quite identified. In other words, we have lots of plausible hypotheses, one or more of which may be true or partially true. As hard as it is to do, sometimes it's better to just wait until all the evidence is in, and the studies have been done. Until then, we can speculate, and we can appreciate the fact of these events, and we can think about their effect, but at least I'm going to say about what caused it: I don't know.
4. Speaking of things I don't know...my impression is that there's been an awful lot less of this in U.S. House races, and in gubernatorial contests. However, I don't actually know whether that impression is correct. If it is, it's pretty interesting, and I'd like to know more about it. My guess would be that it says something about the reach of the national conservative "insurgent" establishment -- that is, these cases only happen when national groups choose to target specific seats, and they can't really handle more than a few at a time. If, on the other hand, there have been lots and lots of purist candidates knocking off seemingly better-qualified opponents at lower levels that just haven't received much attention, that would suggest either that local grassroots groups can manufacture these events, or that national groups have a much greater capacity.