DONALD TRUMP, an American presidential candidate, denounced it as “a terrible deal”. Another, Hillary Clinton, does not think it meets “the high bar” that should be applied to trade pacts. Yet proponents of the Trans-Pacific Partnership (TPP), which encompasses 12 countries in Asia and the Americas, including America and Japan, herald it as the biggest multilateral trade deal in 20 years, which will “define the rules of the road” for international commerce. Which is it?

TPP will apply to 40% of the world’s economy. For American exporters alone, 18,000 individual tariffs will be reduced to zero. Much the same will be true for firms in the other 11 members. Even agricultural barriers, usually among the most heavily defended, will start to come down. Foreigners will gain a toehold in Canada’s dairy sector and a bigger share of Japan’s beef market, for example. Some of these reductions will be phased in lamentably slowly, however: American tariffs on Japanese lorries will last another 30 years.

Tariffs in the region were not that high to begin with, though. More important is TPP’s effort to free trade in services. These are not usually subject to the same impediments as, say, agricultural or automotive imports; instead they get tangled up in beyond-the-border rules, such as customs, visas and licensing. TPP promises greater access to markets for more service providers, which over time should provide a boost to productivity.

In spite of scaremongering on the left, the deal does not obviously exalt the interests of big business over those of lowly consumers. For instance, under pressure from Australia, Chile and Peru, America shelved its demand that certain drugs be protected from generic competition for at least 12 years, settling for five instead. In the same vein, TPP’s dispute-settlement mechanism explicitly bars tobacco firms from claiming compensation for public-health rules that harm their business.

To mollify unions and other likely opponents in richer countries, several of TPP’s 30 chapters are devoted to protections for workers and environmental safeguards. There are clauses that attempt to slow deforestation and overfishing. All parties will also be compelled to follow the International Labour Organisation’s basic principles on workers’ rights. They will be required to set a minimum wage and regulate working hours. Vietnam will have to allow unions independent of the Communist Party. Such commitments will be enforceable under the treaty’s dispute-settlement mechanism.

TPP also attempts to limit the extent to which governments can favour state-owned enterprises. Although there are lots of exceptions, this is quite a concession for the likes of Malaysia and Vietnam. According to Matthew Goodman of the Centre for Strategic and International Studies, a think-tank, “The White House feels this is a big one. It validates their definition of TPP as a 21st-century agreement.”

Since the fine print of the deal has not yet been published, and since tariff reductions form so small a part of its measures, it is very difficult to estimate how big a boost TPP will provide its members. The Peterson Institute for International Economics, another think-tank, estimated that it would boost the world economy by $223 billion by 2025. The greatest impact will be felt not in America, but in the less developed members. The study estimates that Vietnamese GDP could rise by as much as an additional 10% over the same period.

In the long run, TPP’s impact will depend on whether or not its membership expands, as it in theory might once the deal is up and running. South Korea, not one of the original 12, is pressing for swift accession. The crucial question is China. Many think America only pushed TPP forward in order to bolster its influence in Asia and counter China’s. But TPP’s economic significance will be severely curtailed if it does not include the country that lies at the heart of almost all Asia’s supply chains. China may now step up its push for a broader regional free-trade deal, built in part on TPP, says Jeffrey Schott, a former American trade negotiator.

Until TPP is ratified by its 12 original members, such talk is premature. This process should be straightforward in places like Japan and Singapore, where the ruling parties have commanding majorities. But Canada faces a knife-edge election on 19th October. One of the three main parties is campaigning against the agreement, arguing that it will kill farm jobs.