… is from page 98 of the original edition of James M. Buchanan’s and Richard E. Wagner’s insightful 1977 book, Democracy in Deficit: The Political Legacy of Lord Keynes (links added):
To remain in office, the politician need not meet the demands of all constituents. Instead, he need satisfy only a required subset, usually a majority. Because majority coalitions shift as among different policy issues, the behavior of the politician who seeks to maintain majority support need not reflect properties of rationality normally attributable to an individual who chooses among private alternatives. This feature of democratic politics has been exhaustively discussed by social scientists since Kenneth Arrow formally proved what he called the “impossibility theorem.”