Mr Critchley is more significant than Sir Richard and the Eight, because he is on the One Nation - one might almost say New Labour - wing of the Tory party. There are many other Conservatives who it can be imagined would be more comfortable with Tony Blair as prime minister than Michael Portillo. And after today, a Tory defector can have the added glory of a paragraph in history to the effect that they wiped out John Major's majority. For Mr Major's majority will be wafer thin following his party's expected defeat in today's Staffordshire South East by-election.
Of course, James Callaghan defied parliamentary arithmetic and electoral gravity for three years after he lost his majority. (Who now remembers the role played in this by the defectors of 1976: John Stonehouse to the "English National Party" before his expulsion from the Commons; John Sillars and John Robertson to the Scottish Labour Party?)
But most of the time, Mr Callaghan had the support of the Liberals in a formal pact. This did not stop his government suffering embarrassing defeats - even being forced to cut income tax in 1978 - but ensured he could not be brought down. He survived fewer than six months after the Lib-Lab pact ended in the autumn of 1978.
As Mr Major takes his penultimate step to minority government, the record of the past and the predictions of the future do not suggest that he can remain in power until his preferred date for the next election, 1 May 1997, still just over 12 months away. Even if there are no defectors longing for historical paragraph status, the actuaries tell us that one Tory MP can be expected to die every three months. This means the majority would disappear by July and be turned into a minority by October. It would take two deaths because by-elections are nowadays postponed for as long as possible; it is the iron law of this parliament that the Government cannot win by-elections.
Almost the only thing that matters in Staffordshire South-East today is the size of the swing to Labour. It is likely to be less than the postwar record 29-point swing posted by Ian Pearson in Dudley West - in many ways a similar Birmingham overspill constituency - in December 1994. If the swing is greater, then the Conservatives are in very serious trouble indeed. It would suggest that the scare over BSE in beef had destroyed the Government's patient efforts to rebuild its trust on the back of one of the more virtuous recoveries in recent economic history.
But the real question is likely to be how far the movement to Labour falls short of this Blair honeymoon benchmark. If Labour wins the seat with a majority of only a few thousand votes, it could suggest that the traditional link between the performance of the economy and the popularity of the government was finally being restored.
A fuller test of public opinion will be offered in the local elections on 2 May - which give about half the population (and not including London, Scotland and Wales) the chance to deliver the Tories' annual drubbing. The issue for the past three years has been not whether the Tories will lose hundreds of seats, but did they do better or worse than last year?
Whatever the outcome today and in three weeks' time, it is unlikely to persuade the Government to bring forward its plans for the next election. As in 1991, Mr Major is convinced that the longer he plays the election, the better his chances are. The fact that he faces a fundamentally more difficult task than five years ago does not deter him from trying to make the best of his hand.
His government is not yet under immediate threat. Mr Critchley may not like the idea of the Tories promising a referendum on the single European currency, but he is unlikely to be offered the chance of a Commons vote on the matter. Next week, Labour intends to stage a debate on the sale of Railtrack. But, despite yesterday's surprise resignation of the franchise director for the privatised railways, Roger Salmon, there are no signs that the sale worries Tory MPs enough to bring down their government.
The question that dominates Westminster is: When Major loses his majority, what then? As in the 1970s, Northern Ireland suddenly moves to the heart of the politics of the UK. Mr Blair has pursued a policy of bipartisanship from the moment of the IRA ceasefire in August 1994, and has continued it since the ending of the ceasefire earlier this year. He has said he would not try to bring the Government down over Northern Ireland, and this has not changed after the tempered criticisms of the Government made by Marjorie Mowlam, Labour's Northern Ireland spokeswoman, on Tuesday.
If there is trouble for the Government over Northern Ireland, Labour will not be making it. But the Ulster Unionists might. It was Jonathan Caines, recently special adviser to Northern Ireland Secretary Patrick Mayhew, who suggested in a leaked document that the Unionists might see no reason to sustain the Government this autumn. Their voters are just as disillusioned with this government's record on the economy and public services as voters in the rest of the country. And if they are satisfied that Mr Blair is at least no more hostile to their interests than Mr Major, why wait?
Relations between the Government and David Trimble's Ulster Unionist Party were also soured by angry exchanges over the vote on the Scott report. All nine UUP MPs turned up to vote against the Government and it was only Tory Rupert Allason's last-minute switch that carried the day by one vote. But the UUP resented accusations that they offered a deal in return for their votes. Since then John Taylor, UUP deputy leader, has been making recklessly aggressive noises about how little his party cares whether Mr Major survives or not.
Just the kind of friends Mr Major needs as he clings to the edge of his parliamentary precipice.
On the grid, ready to go
Conservative: Only 293 of 641 non-Northern Ireland seats have candidates.
Labour: All but 16 candidates have been chosen.
Liberal Democrat: Just over 300 candidates are in place.
Conservative: The key agency is M&C Saatchi, led by Maurice Saatchi, veteran of the past four Tory campaigns.
Labour: BMP-DDB Needham, led by Chris Powell, brother of Tony Blair's chief of staff, Jonathan Powell.
Liberal Democrat: Knight Leach Delaney, the Delaney being Paul, whose brother Barry once made party political broadcasts for Labour.
Conservative: Chairman, Dr Brian Mawhinney, and Deputy Prime Minister, Michael Heseltine, are the main axis. Former Express journalist Charles Lewington presents.
Labour: Centrally, Peter Mandelson, Tony Blair's closest adviser, who ran Labour's 1987 campaign. But formally, Gordon Brown, John Prescott and Robin Cook lead the campaign, presented by Alastair Campbell.
Liberal Democrat: Campaign chairman, Lord Richard Holme, gave up fighting Cheltenham just before the Lib Dems won it. Preparing the campaign is Alan Leaman, who says this time there will be "more of a team feel".Reuse content