With the disappearance of the Tories' Commons majority, a no confidence vote would leave the nine Ulster Unionist MPs with the power to decide the Government's fate and place them under huge pressure to ditch the Prime Minister.
Already they are under attack in Northern Ireland from the rival Democratic Unionist Party, for supporting Mr Major's administration. Ian Paisley's DUP would certainly exploit any role played by David Trimble's nine-strong UUP in saving the Prime Minister. That could cost the UUP seats both in the general election and in May local elections in Northern Ireland.
Today a poll in the Wirral constituency indicates that Labour are indeed heading for a resounding victory. The MORI poll in the Mail on Sunday gives Labour a 19-point lead, on 54 per cent, with the Tories on 35 per cent and the Lib Dems trailing on 10 per cent. If this were repeated at the poll itself, Labour would romp home with a 7,400-vote majority.
Asked about a motion of confidence last week, a senior Labour source said: "We have made no moves so far because we have not been certain of the numbers. When we are, that's when we will move. We want to get rid of the Government as soon as possible."
If the Opposition wins a convincing victory in Wirral South, as expected, failure to table a motion of no confidence could be seen as a sign of timidity.
Worries about a parliamentary defeat following from the by-election lie behind the Prime Minister's determination to keep the early election option open. Evidence of a deck-clearing operation last week includes:
t Moves to rush out the Conservative Campaign Guide, the "bible" for Tory candidates that spells out the party's record and its policies. Initially due to be available on 14/15 March, this will be ready in a week's time.
t Plans to wind up the political talks in Northern Ireland within the next two weeks.
t Failure of Government whips to discuss any Commons business beyond the next two weeks with the Ulster Unionists.
t Advanced general election preparations at Conservative Central Office, including claims that planes have been chartered.
Mr Major has twice declined to state categorically that the by-election will take place as scheduled on 27 February.
Tory strategists concede that, while the predicted loss might not wreck their election prospects, combined with a vote of no confidence it could prove fatal.
Conservative Central Office has lined up two platform speeches for Mr Major in the next weeks, either of which could be used to launch a campaign proper. He addresses the Welsh Conservative Party conference on Friday, then, a week later, a local government conference in Birmingham. Tory sources see this second speech as an increasingly likely springboard.
The two parties have the same number of Commons seats following the death last month of Labour MP Martin Redmond. Only if Labour wins in Wirral South will the Conservatives again be a minority.
Campaigning continued yesterday when Mr Blair told his party's local government conference he would reverse "the mismatch between spending on education and unemployment".
By-election report, page 4