… is from page 176 of volume 1 of The Collected Works of James M Buchanan , the 1999 volume Logical Foundations of Constitutional Liberty ; specifically, it’s from Jim’s 1987 essay “Keynesian Follies” (footnote excluded; links added):
Ordinary politics in democracy prevents implementation of the policy norms emerging from the Keynesian theory of macroeconomic management, independently of the analytical difficulties that have been previously noted. That is to say, even if there should be no problems inherent in the Keynesian explanatory model, the attempt to stabilize the macroeconomy by compensatory budgetary adjustment would have failed. In ordinary times, political decision makers, ever responsive to constituency pressures, are driven by a natural proclivity to expand rates of spending and to reduce rates of taxation. The Keynesian policy model seemed to offer an intellectual-moral argument for expanded public spending financed by debt (or money). But ordinary politics fails almost totally when the other side of the Keynesian policy norms are required for macroeconomic purpose. Political decision makers cannot increase taxes so as to generate compensatory budgetary surpluses. The bias toward deficits  emerges directly from the most elementary application of public choice  principles.