These are the deal-busters and deal-makers at the December 7-18 conference, say analysts:

-- sharing out the burden between rich countries and emerging giants of curbing the fossil-fuel gases that stoke global warming;

-- agreeing early figures on funding to help poorer countries tackle greenhouse gases and their impacts; and

-- the architecture of the post-2012 climate pact.

"There's still a lot of hardball to be played -- and is being played -- in terms of demanding from each side what they are prepared to do to come forward," said Saleemul Huq, a senior fellow at the International Institute for Environment and Development (IIED), a British think tank.

Huq, in a phone interview from London, did not rule out a walkout by developing countries in Copenhagen.

"Some countries are certainly threatening to pull the plug, although I don't know if they will do that in the end."

Last week, the outlook for Copenhagen suddenly brightened, as the world's No. 1 and No. 2 carbon polluters, China and the United States, finally staked out their positions in the emissions bidding game.

At the weekend, though, the mood soured, as other issues emerged.

Four developing giants -- China, India, Brazil, South Africa -- teamed up with Sudan, which currently chairs the Group of 77 developing countries, in forging a united front.

In a secretive meeting among senior officials in Beijing, they reiterated demands for industrialised countries to sign up to deep emissions targets.

They called on rich economies to shoulder the funding burden to help the switch to low-carbon technology and shore up defences against the impact of climate change.

And they stood resolutely by the Kyoto Protocol, the world's only legally-binding emissions-curbing treaty, whose present round of commitments, expiring at the end of 2012, would cut rich nations' emissions by around five percent.

For the past two years, the 192 members of the UN Framework Convention on Climate Change (UNFCCC) have been meeting to frame the outcome of Copenhagen.

Yet they have barely touched the core question of what the pact should actually look like.

The US rejected Kyoto, saying its binding emissions curbs were too expensive for the American economy and unfair, as they did not apply to China and other emerging giants that are becoming massive polluters in their own right.

Washington has been pushing for a more voluntary approach, in which emitters would make pledges that would be open to scrutiny but not exposed to tough Kyoto-style penalties.

The European Union (EU), despite saving Kyoto in 2001, has signalled that it wants a new overall treaty, rather than a second round of commitments under Kyoto.

The 27-nation bloc says it would be unfair for the United States to have only voluntary commitments and European states to have binding ones.

The EU's approach would be to transfer key Kyoto provisions to a new treaty that would include the United States. Developing countries, as in Kyoto's format, would still have only voluntary targets, though.

"We've been watching this coming as a very, very contentious issue for some time," said Kim Carstensen of green group WWF.

In his opinion, the EU badly under-estimated the feeling in the developing world, which mistrusts rich nations' pledges on emissions and finance and likes Kyoto because of its tough compliance mechanisms.

"It's a very understandable position," he said. "If you can just drop the Kyoto Protocol, how can we have any trust in a new instrument that you present to us?"

Coming up on the inside as a Copenhagen game-spoiler is whether the envisioned outline agreement will set a goal of reducing global emissions by 50 percent by 2050 from 1990 levels, thus limiting warming to two degrees Celsius (3.6 Fahrenheit).

China and India, notably, are opposed to this. They fear being locked into massive cuts in emissions without having the benefit, available to rich countries, of carbon-market "offsets".

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