… is from page 316 of Vol. 19 (Ideas, Persons, and Events  ) of The Collected Works of James M. Buchanan ; specifically, it’s from Jim’s 1990 paper “The Potential for Politics after Socialism”:
There will be no escape from the protectionist-mercantilist regime that now threatens to be characteristic of the century’s turn so long as we allow the ordinary politics of majoritarian democracy to operate in the absence of adequate constitutional constraints.
And, indeed, in the United States ever since the U.S. Constitution has come largely to be believed to be chiefly a document that enshrines principles of raw majoritarian rule – rule limited only by a handful of specific and enumerated prohibitions on government action – the Constitution fails increasingly to protect Americans from the abuse of special-interest groups (including government officials themselves ).
It’s deeply ironic that the political power of ‘minoritarian’ interest groups swells along with the rise of constitutionally unconstrained majoritarianism. To understand the reasons for this apparent contradiction, consult public-choice research .