The Government and the Opposition have both offered significant concessions to the Unionists in the hope of winning the vote. If Labour wins, it could use it as the springboard for a vote of no confidence in John Major's administration.
On the one side, ministers are set to table plans designed to lift the European Union's BSE export ban on "certified herds" of cattle, particularly those in Northern Ireland. On the other, the chairman of Labour's backbench Northern Ireland committee, Dennis Canavan, seen as pro-nationalist, has been removed from his post in what one source described as an orchestrated operation by the party whips. Meanwhile, Jeremy Corbyn, a like-minded left-winger, has lost his vice-chairmanship amid claims of interference from the party's high command.
The Ulster Unionists, who were initially dismissive of Labour's tactics in calling tomorrow's vote - technically on a motion to reduce Mr Hogg's salary because of his handling of the BSE crisis - are considering their position over the weekend. Labour needs their support to have any hope of winning.
Last night Ulster Unionist MP Ken Maginnis said that he expected his party to vote as a block, with a good turnout of MPs. "I expect that everybody who is able to adjust, as I myself have had to adjust, his diary will be there," he told Channel 4's A Week in Politics.
David Trimble, the Ulster Unionist leader, has already been told of the Government's intention to table its proposals on certified herds in Brussels, probably this week. The plans may be made public in the Commons debate tomorrow.
The Government's concessions - designed to achieve a lifting of the ban in areas with grass-fed herds - will benefit Northern Ireland before other areas and could mean some Ulster Unionists abstain tomorrow, saving Mr Major a big potential embarrassment. Some Unionist MPs are reluctant to risk bringing down the Government at a time when Labour would stand to gain a large majority, so making the Unionists less powerful.
One senior source said the proposals would be tabled "shortly". A minister added: "What more could the Unionists want?"
However, the plans still need to be given the go-ahead by Brussels, and some officials in Whitehall are sceptical of the prospects of lifting the ban before the general election.
For its part, Labour last week gave a unilateral concession to Mr Trimble, with the removal of Mr Canavan as chairman of its Northern Ireland committee, after eight years in the post. His replacement, Norman Godman, won by 17 votes to 12.
The composition of the committee was balanced slightly by the election of Kevin McNamara, the former Northern Ireland spokesman, as vice-chairman. He stood when it was clear Mr Canavan had lost.
One MP said: "The whole thing seemed to be orchestrated from the whips' office. The people who voted against were a combination of Blair loyalists and closet Unionists." Another MP argued it was an act of revenge against Mr Canavan'scriticism of the leadership over Scottish devolution.Reuse content