But these constraints are far from fail-safe. Industries such as steel have huge political clout. And since exports are also shrinking, the free-trade lobby is weaker than usual. Moreover, many companies that buy abroad are loth to speak out for free trade now in case they want protection later. General Motors, a big steel user, has failed to stand up to the steel lobby.
Politicians are unlikely to put up much of a fight for free trade either.
Republicans are divided and see few votes in taking a stand for free trade. Most Democrats sympathise with protectionist arguments. Many in Congress are beholden to trade unions for campaign contributions. Bill Daley, the commerce secretary, is lobbying furiously for a tougher line on imports. Ms Barshefsky is only slightly more measured.