The TUC has written to more than 400 members of the parliamentary Labour Party urging them to support the union's line in tomorrow's Commons debate on recognition in the workplace.
The unions say they have shown "considerable flexibility" in negotiations with the Confederation of British Industry to set up a practical system that does not cause trouble for good employers.
They have accepted that employers who flout the new law will not face penal sanctions; that there will be a three-year moratorium after a recognition claim fails; and that "frivolous" claims can be rejected.
But they are refusing to compromise on the key demand: that Labour honour its election manifesto pledge to grant a legal right to recognition where a majority of the relevant workforce votes in a ballot for the union to represent them.
The CBI argues that this should mean a majority of all those entitled to vote. The TUC insists that it means a majority of those actually voting.
"Simply splitting the difference with the CBI does not provide a way forward," says the TUC's briefing note to MPs. "We are not differing about the means of achieving a shared goal, but disagree on principle. Their points aim to make the law weak and undermine the manifesto pledge."
There is considerable disquiet among Labour back-benchers that the Government might renege on its pledge, preferring to cultivate its new friends in the business community rather than its old comrades in the TUC. A back-bench revolt threatens, on an even bigger scale than the rebellion over lone parents.
The unions are pushing for a bigger-than-usual turnout in tomorrow's debate on an Opposition motion calling on the Government not to change the law, or to back the CBI strategy if the law must be changed.
Union leaders met Tony Blair to press their claim last week, and they are to meet Margaret Beckett, President of the Board of Trade, for further talks on Wednesday. They will then see the Prime Minister again after Easter, before convening a national TUC conference on 6 May.
A White Paper on the reform of labour law is to be published next month or in June, and the Government has promised that legislation enacting its manifesto pledge will feature in the Queen's Speech in November.
The TUC adds that if the CBI version of the promise - counting abstentions as votes against - had been in operation in the general election, only 14 MPs would have been elected - no Conservatives, no Liberal Democrats, no Nationalists and no MPs in Scotland. "We see no need for anything other than a simple majority in a ballot," says the TUC briefing document.
The Tories have tabled their Commons motion to maximise Labour discomfiture over rights at work. The shadow trade secretary, John Redwood, said yesterday: "Labour must be given an opportunity to sort out their family row in public.
"They are at sixes and sevens over whether to reward the TUC for their past support, or to encourage the CBI for future support. The longer Blair dithers, the bigger his richly deserved parliamentary revolt will be."