As Rich notes below, there has been a little confusion about the discussion of covering preexisting conditions in the Republican Pledge. Early on yesterday, some people (like Peter Suderman, who knows his stuff and who has since offered a correction) suggested that the Pledge proposed a ban on preexisting-condition exclusions like the one in Obamacare — a ban which would require an individual mandate. But what the Pledge actually proposes is a system of high-risk pools combined with protection from preexisting-condition exclusions for people who are continuously insured (basically an extension of the protection that has existed in federal law since 1996). It does not create an added incentive for healthy people to avoid getting insurance, and so absolutely would not involve an individual mandate.
This pro-market (and far cheaper, though not cheap) approach to the preexisting-conditions issue builds on the great work of conservative health-care wonks like Mark Pauly (of the University of Pennsylvania), James Capretta (my colleague at the Ethics and Public Policy Center), and Tom Miller (of AEI).
Capretta and Miller laid out how this would all work, and also provided an excellent overview of the preexisting conditions problem (the scope of which is far smaller than the rhetoric of many liberals implies), in the summer issue of National Affairs, here. I gather that idea is the basis for the Pledge proposal. It’s a very good alternative to the path laid out by Obamacare.