Most candidates face a challenge going from a primary (where they have appealed to their base) to the general election (where they must offer more centrist message). John McCain is fortunate in that his primary message is his general election message. No significant policy readjustments appear needed to capture moderates and independents. Indeed, the pressure on McCain has been to go back and become more conservative to satisfy disgruntled elements on the Right–a plea he so far has ignored.

The challenge McCain does face is beefing up his domestic policy positions. Many voters suspect, I think, that McCain lacks interest in domestic matters. The flap with Mitt Romney over whether McCain admitted his lack of economic expertise only re-enforced this concern. On Tuesday, McCain began to address this problem with a detailed speech on the housing crisis. He generally got positive reviews from market-oriented commentators and avoided sacrificing conservative economic principles in the rush to soothe nervous voters/home owners.

This speech cannot be an isolated set piece. In his stump speech and town hall meetings McCain needs to talk fluently and frequently on the economy, free trade, and healthcare. (As to the latter, he actually has a very interesting proposal that addresses affordability and access to health insurance without a government mandate.)

His choice of a VP might also help. While McCain cannot appear to be subcontracting out his economic policy responsiblities, it would be wise to select someone with economic expertise, especially in the regulation of financial institutions or in budget and trade policy.

But make no mistake: McCain must convince voters he is knowledgable and engaged on domestic policy. Having substantially assisted in promoting and defending the surge, he may now be a victim of its success. Voters are turning their attention away from Iraq (which is receding from the front pages, except for the occasional acknowledgment of “grim milestones” like the 4000th casualty last week) and looking for answers on domestic issues.