Matthew Yglasias worries that a Christine O'Donnell win over Mike Castle in Delaware's Republican Senate primary might push America's political culture to the right. That is based on some shaky assumptions (like the one that the two parties are probably fated to divide time in power about equally - a possible outcome, but not at all certain), but it is worth thinking about conditions a conservative primary challenge against moderate Republicans might push the Republican Party to the right in the short-term, but shift the political culture to the left.
Within our present context, a "moderate" member of the Senate or House of Representatives (of either party) is usually an opportunist looking for attention and continued reelection to Congress. That doesn't mean they have no principles at all. They would probably continue to support democracy against Nazi or Communist tyranny even if support for dictatorship polled in the low fifties. But within the current parameters of American politics, whatever principles they might have don't usually come into play in a decisive way. Whether taxes go up or down five percent, whether there is a health insurance mandate, or whether cap-and-trade passes matter primarily to the extent that they impinge on the moderate's survival and prominence. They will vote with their party when it is popular to do so and ostentatiously break with their party when the polls (either nationally or in their own constituencies) go sharply in the other direction. When the polling is ambiguous on an issue of great public concern, they will usually demand bribes for their states or districts, as well as private and public stroking in return for their supporting a watered down version of their party's agenda. They are a vain and infuriating bunch and no sentiment should be wasted on them. Conservatives should seek to replace "moderate" Republicans with conservative Republicans whenever possible.
But in cases where a Republican conservative can't win (either because the constituency would not elect one or because the more conservative candidate is fatally flawed), a Republican moderate is usually preferable to a liberal Democrat. Republican moderates can usually be counted on to vote for the popular parts of the Republican agenda whereas a liberal Democrat would be a vote against. Moderates (of either party) can usually be induced to vote for policies that are supported by the majority of their party but are only marginally popular (or marginally unpopular.) Winning their support can often be a painful, irritating process, but the presence of a moderate of your party rather than an ideologically opposed member of the other party can make the difference between a major policy shift or a stalemate. Imagine how different the health care issue would have played out if Democrats had opted for ideological purity and nominated losing liberal Senate candidates in non-left-leaning constituencies like Indiana, Louisiana, and Nebraska and instead of "moderate" Democrats like Evan Bayh, Mary Landrieu, and Ben Nelson, we instead had conservative Republicans. I'm not too concerned about about the short-term consequences of Republicans losing the Delaware Senate seat. For 2011-2012, it is likely to be about the same stalemate whether the Senate Republicans have 49, 50, or 51 votes. But it would be a shame if having a liberal Democrat Senator from Delaware rather than a moderate Republican prevented the passage of a major conservative reform of health care (or taxes or whatever) on the off chance (or maybe a little more than an off chance) that a Republican President was elected in 2012.