Numbers versus instinct.

No contest, you might think.

After all, all recent Lib Dem leaders - David Steel, Paddy Ashdown, Charles Kennedy and Menzies Campbell - have all reached political maturity in opposition, if not antagonism, to the Tories

The deputy leader Vince Cable has hinted that he would have stayed in the Labour Party - he once co-wrote a pamphlet with Gordon Brown - if he hadn't moved to London where the so-called "loony Left" had taken the party over.

Many of the party's leading veterans and founders - Baroness (Shirley) Williams and Lord (Tom) McNally - were senior figures in the last Lib-Lab pact.

Since the party failed to make a northern breakthrough against Labour, more of its MPs and activists see the Conservatives as the enemy.

So, why can you not assume that Nick Clegg will march into Gordon Brown's cabinet today?

The answer is his own declared objective to form a "strong and stable" government. A Lib-Lab coalition would not have a parliamentary majority. Ah, said Paddy Ashdown this morning, but the Tories would never be able to form a blocking majority with the SNP, Plaid and the Unionists. They wouldn't need to. The thing that defeats, exhausts and depresses minority governments is rebellions on their own side, the insistent demands of their allies and the passion of their opponents to run them ragged. Ask John Major or any veteran of the Wilson or Callaghan governments and they'll tell you that.

Now, if there is enough commitment on the Labour side to make this work all those inevitable problems of a minority government could be lived with to build "a progressive alliance".

That, though, is what Lib Dems are asking themselves this morning. Do David Blunkett and John Reid speak for many Labour figures who question the legitimacy of this arrangement and would rather go into opposition and fight the Tories. How many agree with Jack Straw's private doubts about the wisdom of this coalition? Will Ed Balls be more interested in making the new politics work or in securing the leadership of the Labour Party by standing up for his party against their new bedfellows?

As one Lib Dem put it to me - Gordon Brown going was a key that unlocked the door to a deal but it unlocked another door - to a leadership contest in which some candidates will position themselves as hostile to a deal. Another senior figure confessed that if you stripped away the party labels we'd have to go with the Conservative offer as it was so much more substantial.

So, the sincerity with which Labour enters their negotiations this morning and the reaction of the wider Labour movement will determine whether by the end of today the Lib Dems follow their hearts and ignore those nagging doubts in their heads.

Update 12:03: More and more Labour MPs are saying privately that the proposed Lib/Lab deal cannot work.

To understand what this could mean in practice in the Commons, picture the following scene.

It's an hour before midnight. A group of Labour MPs is in the Strangers' Bar in the Commons. It's the latest in a long line of late-night sittings forced on them by the guerilla war run by the Tory whips' office in which they refuse to co-operate with the government. A Labour whip walks in and tells his colleagues that a vote is imminent on a vital amendment to the home secretary's cherished Bill. "You mean Clegg's daft plan?" one asks, downing his pint. "He's not one of ours, is he?" another adds sarcastically while ordering another. "My voters didn't vote for him as far as I can recall," adds a third bitterly.

The whip runs upstairs to tell his boss. The chief whip has just seen dozens of Tory MPs returning from long relaxing dinners - after all, none of them has any government responsibilities to worry about. "I thought you said they'd all gone home," he says to one of the junior whips. "Where are the Nats?" he asks another, to be told that after the chancellor refused to back the New Scotland Growth fund, Alex Salmond's boys chose to take early flights to their constituencies. "What about the Unionists?" he shouts, only to be told that Nick Clegg has offended them with his latest liberal utterance.

The home secretary's PPS is called for and is told to pass him a note warning him that sadly the government hasn't got the votes for his Bill. Nick Clegg pulls out of his pocket the dog-eared photocopy of page 120 of his mentor Paddy Ashdown's diaries. Underlined in red pen three times is one line he wished he'd paid more attention to: "A hung Parliament would not be a dream. It would be a nightmare."

PS: I'm well aware, of course, that much of what I've described could also apply to an uncomfortable Tory/Lib-Dem coalition which was resented or rejected by backbenchers of both parties. The big difference, though, is that it would not be a minority government. It would have a significant Parliamentary majority.