Risky. What a tribute to Ming (one thought) that with this leader at least, the risk could be taken. Thirty years ago a cardboard Jeremy Thorpe, acquitted though he was of conspiracy to murder, would have drawn admonitory messages to the trigger finger. A cardboard Paddy Ashdown might have suffered indignities in the trouser area. Charles Kennedy’s right elbow and throat would have been festooned with warnings. But advice in Sir Menzies’s case was clustered around the ears, lips and manly chest.

Confronted by this two-dimensional proto-Ming, I pulled out a ballpoint pen and briefly contemplated affixing a note of my own. Six words would have sufficed. “Your Party could tip the balance.” Where to stick it? The index finger, at its very fingertip, would have been my choice.

The thought behind the note was prompted by a presentation given by Michael Simmonds of Populus (pollsters for The Times) and my colleague Tim Hames to Liberal Democrats at the paper’s fringe meeting at Brighton on Monday. Mr Simmonds had shown how problematical the polling figures look for the party, with support hovering just above 20 per cent. Tim explained how difficult this made it for the Lib Dems to pitch their electoral appeal. Both were persuasive and our Lib Dem guests must have felt caught in something of a pincer movement between them.

But a way out of the pincer’s jaws occurred to me near the end of Hames’s speech. Until then I had agreed with almost every word he said. He had been explaining how, though after the next election a hung parliament might be pregnant with possibilities for a third party, it was difficult to campaign for such a result. “Vote for a hung parliament!” Tim pointed out (to rueful laughter from delegates) isn’t much of a rallying cry. He was surely right. Nor, he added, was “We could hold the balance” much of a bumper sticker.

It was here I disagreed. I suspect the idea might make rather a potent bumper sticker. If I am right, the Labour Party at Manchester in the week ahead, and the Tories in Bournemouth after that, need to take care.

Let me explain. As a Conservative parliamentary candidate I have fought two general elections, in 1979 and 1983, against a credible Liberal challenge. We won both times. I learnt some useful lessons on the doorstep, faced with a voter inclining to the Third Party. My teacher was a wily Tory agent, the late Al Adam. “Don’t argue with them,” he said. “You’ll be wasting your time. The Liberals have a good name around here and the candidate’s respected. Instead, kill them with kindness. Now, repeat after me . . . ‘You’re thinking of voting Liberal? I don’t blame you at all. I respect Peter Worboys, he’s worked hard and I’m sure he’d be a conscientious MP. And his party has plenty of interesting ideas — I hope to argue for some of them within my own party.

“ ‘But the fact is, the Liberals can’t win. Even if they win in West Derbyshire they aren’t going to be the next Government, or even the next Principal Opposition. This election is about choosing the next government, and it’ll either be Tory or Labour. That’s the choice the country has to make. So if it’s a choice between the Conservatives and Labour — and even if you aren’t mad-struck on either of them — don’t you think you need to think about which you’d prefer?’ ”

I’m not wholly proud to say (for its logic is questionable) that I found many Liberal-inclined voters more or less persuaded by this argument. In one election I got more than half the votes cast, though it was unlikely that an absolute majority of the West Derbyshire electorate positively wanted a Tory government.

My doorstep routine was the argument Labour and Tory candidates always fall back on when faced with a strong Liberal Democrat challenger. It has always been potent. But rarely could it have looked more so than it will look next time. The outcome really will seem to be in the balance, for the coming general election will be one of only two since October 1974 where nobody will know who is going to win. And it is unlikely that the Liberal Democrats will look hopeful of anything better than a poor third place. The squeeze will be on them.

In which case, Ms or Mr Lib Dem parliamentary candidate, feeling that squeeze, try this on the doorstep:

“Look, let’s be honest. You’re right. My party is probably not going to lead the next government. But if you read the papers you’ll know that it’s highly possible that no party will get an overall majority. If so then either the Tories or Labour will have to talk to us Liberal Democrats about how far we could support them as a minority government.

“Our aim will not be to wreck such a government, but to civilise it. That means locking out the extremists on either side. If Mr Cameron has to lead a minority government, think of the nuisance all those far-right-wingers in his party, who have never liked him, could try to cause. Look what they did to Mr Major. We Liberal Democrats can insist that Cameron govern as he campaigned: as a moderate.

“If Alan Johnson/Gordon Brown/John Reid has to lead a minority government, the old Left in the Labour Party and the trade unions will try to hold the Cabinet to ransom. We could support the sensible people in the Labour Party against the Neanderthals.

“Liberal Democrats can be Britain’s insurance policy against extremists of Left or Right. If you’re half-persuaded by David Cameron’s “compassionate Conservatism” and “green” conversion, but worried whether he can really practise what he has preached, we’ll make sure he does. If you used to believe new Labour’s promise, but have lost confidence they’ll stick to it next time, we’ll see they do.

“This doesn’t mean we’ve decided to enter a formal coalition, or that either side can expect our support. We’re prepared to support either or neither. It depends on them. You know where we stand — it’s in our manifesto — and that’s the way we’ll be using the influence a narrow result will give us.

“And the more of us there are in Parliament, the better. Think about it. We could hold the balance.”

Is that such a bad idea for a bumper sticker? In yesterday’s Times Mary Ann Sieghart argued that the actual process, post-election, of seeking formal coalitions or informal deals was fraught with problems, and that discussing (pre-election) what form these might take was a minefield. She’s right. I’m not saying the strategy I’ve outlined would work in practice, post-election. It would probably end in tears and recrimination.

But it would work for the doorstep, for the media interview, or for the conference rostrum. The invitation to support a force at the fulcrum of politics — to tip the balance rather than join the dead weights at either end of the see-saw — flatters the voter. Lib Dem frontbenchers can make a positive virtue of refusing to speculate on whom they would trade with or what they would trade. They can insist that their own policy compass is clear, and it would be for others to decide how far they were prepared to accommodate themselves to the green, compassionate, moderate sort of administration they would use their influence to secure.

In the fortnight ahead, Labour and the Tories should beware. Any sign of an internal struggle for the soul of either party plays straight into the hands of a Liberal Democrat party pitching to tip the balance. Sir Menzies Campbell’s index finger could become quite a talking point.

Times Recommends

• Exit Clinton. But what a show next!
• Angel of the South: the wrong horse
• Cannabis: who cares if it's B or C?