Q: Which "fantasy constitution" is that?
A: I'm talking, of course, about the one that has no effect whatsoever on the way the world actually operates. Claiming that the "true" constitution somehow prohibits actions Congress has been taking for decades, which have been repeatedly upheld by the courts, and which enjoy popular political support, is just silly.
You're not going to find the law simply by studying a copy of the constitution and other 18th century texts.
The "fantasy constitution" is an apt description for the legal world that conservatives often operate in. It is a description of the constitution as they imagine it could be, not as it has actually been interpreted in the real world by courts, or plausibly might be interpreted by courts. It is advocacy, not law. It is like trying to deduce the nature of modern Christianity solely by reading the Bible. It can't be done.
I came across the term in comments to a blog post (quoted above) discussing constitutional concerns about the Obama health care plan raised by the Independence Institute, which like most Independence Institute proposals, had no real merit. The objections raised would invalidate almost all of the federal programs since the New Deal, including Social Security, Medicare and Medicaid, despite the fact that these programs have survived multiple generations without falling to a constitutional challenge.