… is from pages 145-146 of Razeen Sally’s excellent 2005 article “Free Trade: The Next 50 Years,” which is chapter 10 of the 2005 collection, edited by Philip Booth, Towards a Liberal Utopia?  (links added):
Adam Smith  fortified the presumption in favour of free trade with an explicit political argument . Protectionism is driven by ‘the clamorous importunity of partial interests’ who capture government and prevent it from having ‘an extensive view of the general good’. Free trade, in contrast, tilts the balance away from rent-seeking producer interests and towards the mass of consumers. It is part of a wider constitutional package to keep government limited, transparent and clean, enabling it to concentrate better on the public good.