The National Union of Teachers, once the scourge of Tory education secretaries, briefed Baroness Blatch, the Lords' Conservative education spokeswoman, before she attempted to have new teacher regulations annulled.
The regulations are an essential part of the workload agreement.
The NUT is the only major teaching union not to have signed the deal because of its opposition to teaching assistants being allowed to take full classes.
The union entered into a similar alliance with a group of Labour MPs in the summer, when they submitted an early day motion against the same regulations.
A spokeswoman said: "Our concern is children's education, not where political aspirations of others who are concerned about children's education comes from."
Baroness Blatch used her NUT briefing to argue that the regulations should be annulled because they had been amended, without consultation, from the original draft.
They now allow an unqualified teacher, trainee or instructor to supervise the work of teaching assistants who could take whole classes alone.
She said the regulations "do little to improve the confidence and morale of our professional teachers and the quality of education".
Baroness Ashton, junior education minister, responded that the regulations introduced safeguards in an area where there had previously been a lack of clarity.
Baroness Blatch withdrew the motion before it came to a vote.