Could Mitt Romney brand China a currency manipulator?
Mitt Romney says that if he were to become US President he would brand China a currency manipulator on day one in office.
Let's overlook the question of whether it would be possible to do that formally on inauguration day - the Financial Times's Alan Beattie says not, but he could certainly declare an intention.
But beyond a bit of undiplomatic name calling, what in practice might that mean?
The key US legislation requires the Treasury Secretary to assess whether other countries are currency manipulators.
So far, the regular reports the Treasury produces have not labelled China as such - at least, not since the 1990s.
The most recent report from May this year said, again, that China falls short of deserving the tag.
If China were declared a currency manipulator, all that is required of the Treasury is that it enters into negotiations
But could the US administration go further? Could they impose retaliatory tariffs on Chinese goods?Hidden subsidy?
As a sovereign power with control over its own borders, the US could do it. But doing it legally would be tricky.
Governor Romney does have an idea. It is to instruct the Department of Commerce to take exchange rates into account when deciding whether imports are subsidised, because under World Trade Organization (WTO) rules you can impose what are called "countervailing duties" for some types of subsidies.
This is a situation where the WTO rule book allows countries to act unilaterally, provided that they follow specific procedures.
The US is the leading user of these anti-subsidy measures in the WTO's history and China is Washington's leading target.
The idea behind Mr Romney's proposal is that a currency kept artificially low has the same effect for an exporter as a subsidy.
But it is not at all clear that the WTO's rules would allow a currency policy to meet the definition of a prohibited subsidy.
It is almost certain that if the US did try this approach, China would make a WTO complaint.
There are some other provisions in the WTO rules that refer to exchange rates.
One says countries "shall not, by exchange action, frustrate the intent of the provisions of this agreement".
That seems to be a ban on using currencies policies as a barrier to trade.
If a President Romney wanted to use that against China, he would have to take it up in the WTO's dispute system, where the US would almost certainly have to persuade an independent panel that Beijing had sinned.Political currency
None of the rules that refer to exchange rates has been tested before a WTO panel, so we cannot know what the outcome would be.
But it is possible to anticipate some important arguments that China would surely use in its defence.
The currency has in fact risen substantially against the dollar in recent years - 3% since 2005.
So China would argue its competitive advantage has eroded.
The trade surplus - or strictly speaking, the current-account surplus - has halved in dollar terms since 2008.
China would also probably argue that it has stopped building up its reserves of foreign currency, which can be interpreted as a sign that it has reduced its intervention in the markets.
But what counts politically in the United States is the bilateral trade imbalance with China.
At more than $200bn by the end of August this year, it is still large enough to matter.