Sean O'Grady: Brown out. PR back on the table. Has Clegg snatched victory from the jaws of defeat?

The last time the Liberal party joined the Tories in a coalition the party split three ways

Sky News cut away from Nick Clegg's statement that he was opening formal talks with Gordon Brown slightly late and caught the Liberal Democrat leader turning to the Sky producer and asking: "Is that all right?" Yes, Nick, it is all right.

At 6.20pm yesterday, Clegg turned the historic Liberal dilemma to overwhelming tactical advantage. No doubt he was influenced by a tsunami of protest from his own grassroots, councillors and MPs about the bizarre insanity of a Lib-Con deal, and is now using his power to the best advantage for his party and, in his view at least, the national interest. With no great effort he has got Gordon Brown's head on a stick, and showed the Conservatives that they cannot treat the Liberal Democrats lightly.

For one of the more stunningly unreal aspects of the Lib-Con talks is how the likes of William Hague, a man previously given to dismissing "the Liberals" as jokes, refers almost reverentially to his "Liberal Democrat colleagues" and their constructive, positive relationship.

The Cameron-Clegg talks reminded me of the famous David Low cartoon after the Nazi-Soviet non-aggression pact. Hitler to Stalin: "The scum of the earth, I believe?" Stalin to Hitler: "The bloody assassin of the workers, I presume?" Now it is Brown's turn to call the Liberal Democrats, for the first time, by their correct title.

From what one can divine about the meeting of Liberal Democrat MPs yesterday afternoon, they could see the unfeasibility of the arrangement, and baulked, at last, at the prospect of being successively duped, split and then absorbed by the Tories. When Nick Clegg made his fateful campaign pledge to work with whoever got the most votes and/or seats, that was, though democratic, a fundamentally value-free approach. It obliged him, as we now see, to bridge an unbridgeable gap. It did not entirely succeed, and Gordon Brown seized his chance to gatecrash the talks with his audacious offer to pack it in, though not forthwith. Mr Clegg took the hint.

The Liberal Democrats have good reason to mistrust Labour – not least their dishonoured 1997 manifesto promise to hold a referendum on electoral reform – but at least the two parties are both in the progressive camp and have some values in common. Labour is also more desperate.

The Liberal Democrat leadership should place themselves in the Tories' shoes. What would be the ideal outcome for them, long-term? A violent Liberal Democrat split – inevitable with a Lib-Con deal – suits the Tories fine. After that, they might be able to recruit the "Orange Book", market-oriented Liberal Democrats they've been wooing for years. The result would be to reduce the party to a rump. The last time the Liberal party joined the Tories in a coalition, in the 1930s, the party split three ways. It could easily happen again.

More immediately, a Lib-Con deal will propel a Tory into No 10, and into all the key ministerial jobs, for the first time in 13 years. Thus Vince Cable can be allowed the job of chief secretary to the Treasury, but not the chancellorship. Mr Cable can be the man who toils in the undergrowth of Whitehall with his axe, while George Osborne gads around the G20 summits and the rest of it. Thanks, George. Thus, also, Michael Gove can nobly sacrifice his putative role as schools secretary in favour of David Laws, safe in the knowledge that Mr Laws agrees with 90 per cent of his policy anyway. Mr Clegg, Prescott-style, could be appeased with a grand but meaningless title of deputy prime minister, and left to fiddle around with the constitution. But won't Nick Clegg force through electoral reform? What about the Tories' promises?

The sad answer is that the Tories will find a way of dodging their obligations. For them the Liberal Democrats are not partners in power, but enemies to be destroyed – by stealth if necessary, as outright electoral assault has not worked. The last thing they will give the Liberal Democrats is a permanent lock on power.

Any commitments the Tories make on electoral reform will be confined to inquiries and referendums, and the Tory party itself , and possibly Cameron, will not change settled policy. Even though they have offered a referendum on the alternative vote, they might not agree to whip the Tory MPs to support the enabling Bill. And even if they did, lots of Tory MPs might ignore the whip. The sanctions imposed by the Tory leadership for such indiscipline might be light. All this, by the way, will be late in the parliament, and too late for the Liberal Democrats to do much about it.

Now, in their talks with Labour, the Liberal Democrats have the best chance of fair votes in 80 years. They also have Brown's head on a stick (which is certainly what the nation wanted), immediate legislation on electoral reform, and an agreed programme for economic recovery, probably with David Miliband as PM, Vince Cable in No 11 and Clegg as deputy PM, this time with a proper job of political reform to carry though straight away. That, it seems to me, is the fulfilment of the original Tony Blair-Paddy Ashdown "project" of 12 years ago to re-unite the progressive parties. Gordon Brown vetoed that in 1997 and 1998; he has now acted as midwife to its rebirth: a pleasing symmetry.