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WASHINGTON DC -- Why do you think President Obama has blocked a Chinese-owned company from buying some fields full of wind turbines in the Western state of Oregon?

Why, on the other hand, is another Chinese-owned company building a network of 50,000 solar panels on a flood-prone field in Illinois? Come to that, why does the same company employ thousands of American workers in the car components industry, making things that, in some cases, are then exported back to China?

It's all part of an increasingly complex relationship between the world's two biggest economies - a relationship that is at the heart of the US presidential election campaign and will be at the heart of the American foreign policy debate over the next four years.

First, those wind turbines in Oregon. It so happens that the land on which they stand is close to a US military base at which unmanned drones are tested. The White House says having a Chinese company as a neighbour could threaten US security interests, so - for the first time in more than 20 years - the President has used his powers to block the sale. (The company says he has overstepped his powers, and is suing him.)

There are no security problems with the solar panels in Illinois, which are being erected in a field that isn't close to anything of any conceivable military significance. So the message from the US seems to be: Chinese investment, yes please, unless we think you could be spying on us.

For several years now, American politicians have accused China of "stealing" American jobs. Low pay rates and unsafe working conditions, they say, make it irresistible for American manufacturers to shut down factories at home and open up instead in China. More jobs for Chinese workers, fewer jobs for Americans.

Now, though, the picture is getting more complicated. Chinese pay scales have been on the up, the currency exchange rate has shifted so that it's no longer quite as favourable to Chinese exporters - and, as with those wind turbines in Oregon and solar panels in Illinois, Chinese companies are investing in the US, buying American companies and hiring American workers.

So what does all this mean for the future relationship between the two global giants? In a special programme which we hoped to broadcast last night, but which will run tonight instead, from the here in Washington, a panel of distinguished guests, including two former US ambassadors and one of China's leading strategic thinkers, discuss the challenges and opportunities that will face whoever wins next month?s presidential election.

They don't agree on everything, but they do agree that the relationship is undergoing a profound transformation. Quite apart from China's meteoric economic growth, which means that within the next few years it's likely to have overtaken the US as the world's number one economy, it is also about to appoint a new generation of leaders, and they, according to Professor Yan Xuetong, will want to develop "a new type of relationship". What that means exactly isn't entirely clear - although he does say it won't be anything like the old Cold War relationship between the US and the Soviet Union.

Will China go on investing in American industry? Yes, he says, because it makes good commercial sense. Should the next US president be cautious about encouraging such investment? No, say the former ambassadors, unless there are clear security concerns.

China has been increasing its military expenditure steadily over recent years; the US has been cutting back. And with rising tensions in the East and South China Seas, where Beijing is determined to protect what it regards as its essential strategic interests, there are inevitable concerns about the potential for naval clashes between China and one of America's regional allies, like Japan or the Philippines.

So yes, there is clearly the risk of trouble ahead. But what our panellists do agree on is that leaders in both countries are anxious to avoid them wherever possible. Increasingly, the relationship between Washington and Beijing is becoming a relationship between equals - and that's something America isn't yet used to.

You wouldn't know it from the way the presidential campaigns are talking about China, but maybe we shouldn't pay too much attention to anything they say in the run-up to a closely-fought election. The key will be what they do after Inauguration Day next January.