Staunch conservative Christine O’Donnell is challenging Rep. Mike Castle (R-DE at large) for the Republican nomination for the Delaware Senate seat formerly held by Joe Biden. Despite Castle being the establishment choice with a good chance of winning in a blue state, O’Donnell has lined up impressive endorsements from powerful conservative organizations, including the Susan B. Anthony List, Tea Party Express and Concerned Women for America. Seeing conservatives wrestle with which candidate is the best choice for advancing their policy goals is a great way for the progressive community to understand the policy and economics of primary challenges.

Roll-call votes

Let’s assume O’Donnell would vote close to 100 percent on the conservative agenda in the Senate if she’s elected. Castle, on the other hand, would be a less reliable vote. He is one of the most liberal House Republicans, and if elected to the Senate, he might even go slightly to the left of a few conservative Senate Democrats. Castle would likely break with the party often. I’d put him at best as a net 50 percent conservative vote. He’d support Republicans on important items like committee assignments, but might break on topics sure to anger conservatives, like cap and trade. Assuming that both have an equal chance of winning the general election, O’Donnell is a much better choice for conservatives.

Electability

The problem for conservatives is that the candidates are not equally electable. Castle is basically the incumbent, having held the at-large spot for years (Delaware has only one House seat). He is heavily favored against Democrat Chris Coons. The Cook Political Report ranks Castle a likely Republican win, Rothenberg Report has it leaning Republican, and Nate Silver currently says Castle has a 64 percent chance of winning.

So, for this experiment, let’s use 64 percent. O’Donnell does not have the same name recognition or history in the state as Castle, who is also a former Governor. Her supporters acknowledge that she does not poll as well as Castle, but she still had a small lead over Coons in the latest Rasmussen poll. Delaware is a blue state, but we have recently seen Republicans win in very blue states like Massachusetts. Given the polling numbers and the general mood, let’s give O’Donnell a 36 percent chance of winning over Coons.

Conservative voting vs. electability: a two-variable equation

With just these two crude variables, O’Donnell is the better policy-economics choice for movement conservatives. A 36 percent chance of electing a 100 percent conservative (36 percent chance of eventual conservative votes) can be seen as better than a 64 percent chance of electing a 50 percent conservative (32 percent chance of eventual conservative votes). Of course, how you set these variables is important. If you suspect Castle might be more conservative than not, or that O’Donnell has only a 20 percent chance of winning the general election, Castle could be the better choice. Similarly, if you are a single-issue activist and Castle is against your position but O’Donnell supports it, Castle would be a 0 percent chance and O’Donnell would be 100 percent in your camp. So, even if O’Donnell has just a small chance of winning, she’s worth supporting.

Long-term goal of 51 conservatives vs. 51 Republicans

Politics and policy are not as simple as two variables. The long-term goal of activists hoping to implement conservative policies should be to elect 51 (or even 60) hard-core conservative Senators. To get that, conservative activists will need to elect like-minded Senators from some states that are not particularly conservative. If they don’t take advantage of moments like this one, where they have at least a modest possibility of electing a true conservative in a blue state because of an open seat and Republican momentum, it is hard to see how they will ever get 51, let alone 60, true conservative Senators.

Defeating Castle in the primary with a less-electable conservative does slightly reduce the chances of Republicans taking control of the Senate. But that only matters to conservatives if they think a Republican majority filled with several moderates can achieve conservative policy. Progressives have seen first hand that having a large Democratic caucus filled with conservative members definitely does not mean you will get much of the progressive or even the Democratic platform passed into law.

There is value for conservatives in Republicans controlling the Senate, even with a large number of moderates among them, at least to block Democratic policy. But that’s overkill in this environment. With the filibuster in place, a mere 41 to 44 true conservatives can already be extremely effective at stopping Democratic and progressive reform.

External factors

Several external factors dramatically shape the value of a primary challenger. Resources are finite, so money spent on O’Donnell means it’s not available in other places where it might be more effective. A bruising primary fight that leaves Castle a weakened winner is, in many ways, the worst of all worlds. There is also the possibility that, having won the primary, Castle will think he has nothing to fear from the conservative movement and owes it nothing. As a result, he’ll vote in an even more liberal way. There’s even a slim chance he could end up pulling a Charlie Crist or Arlen Specter and leave the GOP while in office.

Dynamic of the Senate and future races

Perhaps, though, the most important part of any primary challenge is the psychological effect it has on the dynamic of the Senate and future races. One or two moderate Republicans might not be comfortable bolting to vote with Democrats. But conservatives should worry that a solid bloc of six to eight moderate Republicans in the Senate could give one another enough cover to do so.

Supporting O’Donnell could have an even bigger impact than possibly electing just one more conservative to the Senate. If the fear of primary challenges causes several more moderate senators like Scott Brown (R-MA), Susan Collins (R-ME) and Olympia Snowe (R-ME) to start voting more conservatively, this has a huge multiplier factor. If the primary actually shifts the voting pattern of a half-dozen Republicans to the right, it could be well worth it from a policy standpoint to support O’Donnell, even if she’s a long shot in the general.

You need to look outside for perspective

Understanding how and when primary challenges are effective for advancing the policy of a group of activists is not always easy. There are many factors at play and some are very hard to quantify. How much value different groups put on certain issues and how likely they think a challenger is to win the general election will affect support for a primary challenge.

Understanding these factors and judging them properly can be difficult when you are on the “inside” and emotionally wrapped up in the issues and elections. Progressives should take a dispassionate look at the policy economics at play in the primary challenges of conservative activists, and learn lessons to apply to their own plans.