Legislation that targets currency manipulation might make or break U.S. President Barack Obama’s signature Pacific trade deal. But calling out offenders for currency transgressions is far from a clear-cut exercise.

U.S. lawmakers, companies and unions are trying to use pending trade legislation to strike back at countries they say subsidize their industries through devalued exchange rates. They’re finding themselves entangled in a decadeslong debate about currency valuations that has bedeviled leading institutions such as the International Monetary Fund and the World Trade Organization. The complicated currency calculus is likely to persist well beyond any deal, stymieing attempts to rein in exchange-rate complaints through arbitration or diplomacy.

Some U.S. lawmakers are channeling their constituents’ long-held grievances by pressing to incorporate enforceable currency provisions into the Trans-Pacific Partnership, a proposed 12-nation free-trade deal tying together 40% of the global economy. Legislators from manufacturing-heavy states in particular—such as Michigan, Ohio and New York—worry that TPP members and potential future partners such as South Korea and China could offset any gains made through the pact by depreciating their currencies.

A weaker currency slashes production costs and fuels exports, at the expense of competitors overseas. Some economists and companies, for example, say China’s managed exchange-rate policy over the past decade cost the U.S. millions of jobs as Beijing subsidized its exporters by keeping the value of the yuan as much as 40% below the level market fundamentals would suggest.

“Currency manipulation is the mother of all trade barriers,” said Stephen Biegun, a vice president of international government affairs for Ford Motor Co. Ford, like other U.S. auto makers, is backing lawmaker proposals to punish countries that depreciate their currencies to gain a competitive advantage. The firm is particularly concerned about Japan, one of the largest car markets in the world and a TPP negotiating partner.