By Stephen Mulvey
EU reporter, BBC News, Brussels
Poland shocked EU diplomats with its fierce negotiating tactics even before it joined the club, and after three years of membership neither the ferocity, nor the shock, have gone away.
Angela Merkel faces determined opposition from Poland
Power and money are always the toughest issues for the European Union to solve.
Poland's forceful debut at the negotiating table came when the EU was deciding on things like milk and potato starch quotas for the new member states, which would determine Polish farmers' incomes.
The big issue at this summit, for Poland, is about power, and the weight of its vote when the 27 EU member states take any decisions that do not require unanimity.
Poland is against a new voting system for these qualified majority votes, which would tie voting strength to the size of a country's population.
And since the summit can only take decisions by unanimity, Poland can easily bring the whole thing "crashing down", as one diplomat put it.
Give-and-take is the essence of an EU summit. If a prime minister or president is communautaire he or she makes compromises that allow a deal to be reached for the common good - or at least for the sake of harmony.
The UK and the Netherlands, which have also come here armed with some tough demands know the rules of the game. An opt-out here and there, some changes of wording, a few genuine concessions on both sides, and a deal can be stitched together.
Poland is altogether more unpredictable.
There are even signs that it may start arguing with Germany - which is chairing the summit, and setting the agenda - over World War II.
The proposed new voting system would link voting strength to population, so Poland sees the question of its war dead as highly relevant.
""We are only demanding one thing, that we get back what was
taken from us," Prime Minister Jaroslaw Kaczynski told national
radio this week.
"If Poland had not had to live through the years of
1939-45, Poland would today be looking at the demographics of a
country of 66 million."
Commission chief Barroso wants more efficient EU decision-making
The country's population is currently 38m, compared to Germany's 82m. Since Poland has 27 votes to Germany's 29 under the existing system, Germany would be the biggest beneficiary of the proposed change.
Rubbing salt into the wound, Mr Kaczynski added to his comments about the "unimaginable injury" inflicted on Poland in 1939 that "Poles like Germans, while Germans do not like Poles".
For the last few weeks, he and his identical twin, President Lech Kaczynski, have been talking about "death" being better than "capitulation" to Poland's EU neighbours on the voting question.
Poles were used to fighting alone, they pointed out, for example in the anti-communist underground.
Their claims of being isolated and ignored, or not treated equally, are partly true.
Germany can afford to make concessions on some of the demands from the UK, but it cannot afford to unpick the key parts of the balance of power agreed in the constitution in 2004.
Voting weights, seats on the European Commission, seats in the European Parliament - all of these issues, and others, are linked and there is a risk of the whole deal unravelling if it is reopened.
That is why Germany was slow to even allow the possibility of a discussion of the Polish concerns at this summit.
Poland has not quite been alone in the run-up to the summit. It has had lukewarm support from the Czech Republic, and British Eurosceptics have been cheering the Kaczynski brothers from the sidelines.
They were delighted to see a country sticking up for its own national interests, and threatening to veto the whole post-constitution project.
But having failed to get anywhere with its proposal for voting weights based on the square root of a country's population, Poland is now talking about ways of making it more difficult for large countries to block a qualified majority vote.
As they point out, three of the largest countries, plus one other, can block any decision, whereas it takes a big coalition of small countries to achieve such a result.
But British Eurosceptics are already complaining about a reduction in Britain's ability to block legislation under the proposed new system, and the kind of amendment that Poland is suggesting would make that problem worse.
It is too early to say whether Poland will live up to its threat to veto a mandate for an intergovernmental conference to decide on a new treaty.
Some are predicting a long, drawn-out fight, leading to a possible compromise some time around dawn on Saturday morning.
A more pessimistic forecast is that the summit will finish on time on Friday. As one seasoned observer put it, "How long does it take for the Poles to say No?"