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Forget the fight over including one sentence or another, what goes in the square brackets or the footnotes of the deal.

There are so many countries involved, so many complications in these talks, it's easy to lose the big truth.

This is the best chance David Cameron has of getting a new deal with the European Union - there could always be another summit, more talks, more negotiations but there's no guarantee the terms would get any better for the UK. One senior UK diplomat told me: "The deal is as cooked as it is going to get".

Failing tonight might not just mean the best terms and conditions slip away, it would also be fraught with political risk.

Losing this moment jeopardises his chances of holding a public vote in June. Months more of fractious negotiations would clog up the government's time and focus, and remember ministers believe the longer this process goes on, the harder it will be for them to win the referendum.

There would be more time for those arguing to leave the union to organise their campaigns. By offering this referendum on the basis of a renegotiation he chose, David Cameron has manoeuvred himself into this distinctly risky political position.

He never planned to be the prime minister who wanted to expend significant time and energy on the EU, and he certainly does not plan to be the prime minister whose actions led to the UK leaving the Union.

But what happens in the next few hours may well shape his political legacy.