Not to delve too deeply into Douglas Kmiec’s ratiocination, but his “as an X I believe [A, B, …]” formulation may not reflect the muddled inference that Michael O’Hare suggests. Kmiec could have said “I believe [A, B, …]; the Republican Party is defined by these beliefs, and its platform derives from them. By choosing to identify myself as a Republican, I affirm these beliefs.” That would be clumsier than, but roughly equivalent to, what he wrote.
Michael’s alternative formulation (roughly), “I believe [A, B, …]; the Republican Party better serves the advancement of my beliefs,” says nothing fundamental about what it means to be a Republican. Perhaps the Church of the Subgenius would best advance the policies that follow from his beliefs, even if their basis for doing so were inimical to him. He’d not then be likely to say, “As a member of the Church of the Subgenius…”
It’s been years since I’ve been in a class where anyone’s beliefs or identity entered in to the discussion, but from such I recall that many students’ comments were prefaced by “as a bi-flexible Euro-American with asthma, I believe…” and the like. The difference is that Kmiec’s identities (Republican, Catholic) are assumed and are defined by beliefs (yes, they’re more typically inherited traits, but they needn’t be), whereas the racial, sexual, and other essentialisms of the left are much more pernicious.