My point was that in the case of farm subsidies, it doesn't take any convincing; the people believe these are necessary to maintain an adequate supply of food, and they also happen to think supporting farmers is a good cause. As a rule-of-thumb, the bills that everyone signs/votes for very publicly (with ceremonies and such) are actually vote-winners, steel tarriffs are another good example.

Voters hold many mercantilist and nationalist beliefs (on both sides of the aisle), and policies largely reflect these, not some propaganda-driven special interests.