The problem with such arguments is that, in a multi-party democracy, any policy that can only work when one particular party is in power is probably a policy that cannot work at all. As Mike Rappaport puts it with regards to the Iraq War, “[t]hat one of the two parties cannot be trusted to participate in long term policies for which they will inevitably have some responsibility suggests that those policies should not be undertaken.” Some partisan pundits like to imagine that their party can win a decisive enough victory that they no longer have to worry about the possibility that the opposition will return to power in the near future. But history shows that such triumphs are extremely rare.
To summarize, an effective government program must 1) work well when implemented by real-world politicians, 2) be capable of monitoring by a generally ignorant electorate (or, alternatively, does not require public monitoring at all), and 3) continue to work well regardless of which major party is in power. I don’t believe this definitively proves that libertarianism is right, or even that government should necessarily be a lot smaller than it is today. Although I am skeptical, perhaps these three requirements can be met by a much wider range of programs than most libertarians imagine. Regardless, our debates over public policy would be improved if more of the participants kept these criteria in mind.