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Downtown Pittsburgh feels sleepy, even as thousands of diplomats and journalists turn up, like migrating Wildebeest, ready to stampede across the Allegheny and into town.

Sammy's Corned Beef joint full of seasoned barstool sitters, Hemingway's student bar - like a Methodist youth club with beer, even the nameless joint at the corner of a street with nameless wraith-like drug users hanging around just at the edge of the neon glow. The whole mid-western urban idyll is about to be broken by the arrival of the G20 herd.

There are so many answers being promoted this week you could be forgiven for asking "er, what was the question?"

It is only Tuesday and already a heavily hyped Middle East peace agreement has turned into a grouchy photo op.

Next Hu Jin-Tao announced a series of measures on climate change, hailed as China assuming "leadership" on global warming, but not very clearly taking the world towards a comprehensive agreement in Copenhagen.

In fact. not doing anything other than promising to grow carbon emissions less fast than the Chinese economy.

On top of that we now have Gordon Brown advising the Chinese on how to rebalance their economy. I look forward to reciprocal advice from President Hu on how we should rebalance ours.

So what's really going on? The dominant theme in all these narratives is the need for reciprocal, mutual and therefore multilateral action in the face of strategic problems in the world - climate change, poverty, trade, bank regulation, nuclear proliferation and Middle East peace.

Actually, the sheer volume of horse trading in the run up to this summit shows you these are all linked issues.

If there is one problem that recurs in all these initiatives it is this: the failure of the USA to adjust to a world order that is rapidly changing.

In economics, the Anglo Saxon model's prestige is battered. On climate change, action in the USA - after years of no action - is seen as the key to forward movement.

On trade, well the President signed the last G20 agreement - a moratorium on new protectionist measures - then placed new controls on Chinese tyres.

In the last few days I've been finding out just how hard it will be for President Obama to sell any multilateral action agreed at the G20 to the American people.

First there is the domestic right-wing backlash: it started over the bailouts, moved on to the fiscal stimulus and is now focused on healthcare reform.

From Washington's K-street this can look like the Republicans fragmenting, their popular base getting emotional and ruining the party's chances of forming a moderate electoral alliance at the next election.

But in Pittsburgh it does not look that way.

Obama's radical right-wing opponents are creating an emotional narrative which is becoming a dominant theme on the US news networks. It is, in the pop-psychology parlance, giving people "permission" to get angry about a lot of other things.

As a result the G20 will face a new kind of protester this week: conservatives who wish the G20 would stop doing things and who see multilateral action as a potential threat to American sovereignty.

Second there is the fragmentation of the electoral coalition that brought Obama to power.

He is trying to hold it all together - delivering an under-reported speech to the US trade unions last week clearly designed to boost his radical credentials there.

I've been meeting steel industry workers from Clairton, the setting for Michael Cimino's emblematic Nam-era film, The Deer Hunter. They are in no mood to accept multilateralism either on trade or carbon emissions. They want more trade barriers to protect jobs.

As one put it: "Our plant has a raised a cancer hazard. You get up, go to work, pay your taxes and wonder all the time am I going to live very long after I retire. Will I get leukaemia? And all so that some other country can come along and dump a bunch of cheap steel onto our market?"

The workers I spoke to want any deal on carbon emissions explicitly linked to forcing producers in the developing world to raise environmental standards.

"They will fail," predicted one, triumphantly.

Third, crossing the electoral divide, is race. I keep asking people whether the issues like healthcare and tax and bailouts risk tearing America apart. The surprising thing is the number who say: "It's torn apart already, always was".

And when I probe further what they mean is over race.

Whether Jimmy Carter is right or not over the heckling of Obama, the wider fact remains that people are increasingly prepared to talk about a racial divide opening.

Add to that an incredible emotionalism. Okay, Brits are reserved. To me all US television looks needlessly emotional. But laying aside cultural differences there is a lot of emotion on US TV already and now you've got political movements, above all from the right, prepared to make politics more about emotion than it was.

Then, in a different reality, you get the patrician narrative. The reserved, statesmanlike politicians on the Hill; their reflections in the grey-hair-hosted political talk shows and the finely crafted statements of corporate leaders.

And then, below that, at the level of grass roots, you get a preparedness to emote, shout and scream. One right-wing activist - holding profound small-state and decentralising beliefs himself - complained to me off camera that too many people on all sides are beginning to fantasise about some kind of "showdown" in America, in which all the values of self reliance, states' rights and the right to use 9mm ammunition will combine into some kind of cathartic moment.

"People almost seem to want this to happen," he confided to me, genuinely worried.

Now all this is only "noise", and for the politicians noises off.

But I think in the USA, there is definitely an intensifying cultural conflict that is not reflected in the news but actually mediated via the news. People of different political persuasions increasingly live inside self-constructed media bubbles.

The strong standpoint taken by Fox - refusing to show the president's speech and in turn getting itself boycotted in Obama's latest round of Sunday interviews - countered by the recent rise of MSNBC, with its liberal ranters throwing invective at Obama's opponents.

This is the America I've traversed in the process of writing this. I'm in New York now, inside the UN building. Once the UN symbolised multilateralism, but the global crisis has forced the world's leaders to improvise something more useful for tactical intervention.

The explicit dream of politicians in Europe and America is that the G20 becomes a more permanent horse-trading forum, a kind of insecurity council, with a remit to try and forge deals beyond economic growth and trade.

The whole situation is a product of the grand, spatial forces that are shaping the world: the weakening of American power and the crisis of neo-liberalism, the rise of China, global warming.

What I'm watching - and what's about to descend on the residents of Pittsburgh - is a kind of rolling maul of the world's politicians trying to cope with this power vacuum and improvise new ways of dealing with it.

And outside the calm, air-conditioned UN - almost archaic-looking now, with its fifties era furniture and its media centre with only enough power points for a world where computers don't exist - outside this time capsule there is just this palpable emotionalism, for and against the American President, rising in intensity and leading who knows where?

TONIGHT: Watch my report from Clairton, Pennsylvania - how the real life steelworkers and Nam veterans from the town where the Deer Hunter was set see the G20.