PRUSSIAN hero Otto von Bismarck once noted that politics was the art of the possible.

David Cameron is hoping that his compromises and euro-schmoozing mean he has mastered the art and will make it possible for Britain to stay in the Brussels club with all its imperfections albeit with a reform here and a semi-reform there.

With the Tusk “to be or not to be together” document leaving X Y Z blanks to be filled in, questions fired at No 10 receive a robotic declaration that: significant progress has been made; details still have to be worked out and negotiations continue.

With the Outers in disarray amid resignations, squabbling and threats of legal action, the right-wing press has bemoaned the lack of clear Brexit leadership.

Unusually for the Tory leader, he looks likely to lead the pro-EU campaign in the face of screaming opposition from the likes of the Mail, the Telegraph and The Sun; in England, that is. It might be different north of the border.

While a poll yesterday showed 45 per cent of people wanted to leave, it was only last week another one showed 54 per cent wanted to stay.

Having schmoozed European counterparts in London this week, the PM travelled to Warsaw yesterday. Poland, which has an estimated 800,000 citizens working in Britain, is, naturally concerned about them facing a cut to their income because they are, er, Polish.

But there was a more positive than expected response in Warsaw. Could this have anything to do with the prospect that the welfare curbs, imposed through the so-called emergency brake, would not apply to the 800,000 Poles already here but only to new migrants?

The brake itself could take many months to pull as it would need the approval of MEPs and the reduction in in-work benefits would be “graduated”.

While Mr Cameron wants a full four-year ban, it looks likely that while there might be a ban on in-work benefits for the first year or so, the longer a migrant is here the higher the payments will be.

Yet for all the continental concerns over breaching the core EU principle of non-discrimination, Mr Cameron’s most valuable card is the threat of Britain leaving the EU.

To lose Greece would have been one thing but to lose the UK, set to be Europe’s biggest economy in the next decade, would be something else.

If the UK were to say cheerio, the Brussels club would be much diminished and who could guarantee some others would not follow suit.

Yet Martin Schulz, President of the Euro Parliament, while insisting he wanted the UK to stay, decried the “demanding” Brits and admitted: “Many of my colleagues say behind closed doors: 'Don’t stop a rolling stone. If the Brits want to leave, let them leave.'”

Then, of course, there is the Scottish question. Brexit could lead to Scexit; Scotland exiting Britain.

Bismarck’s greatest achievement was getting all those states, grand duchies and principalities to agree on becoming part of a unified Germany but if Mr Cameron does not succeed in his artful pursuit of the possible, his failure could not just see Britain out of the EU but also Scotland out of Britain.

The stakes could not be higher.