The UK's biggest teaching union is supposed to be non-political, but it made it transparently clear this week that it wants its members to vote Labour.
The NASUWT was scathing in its criticism of Conservative education policies and warned it would fight the party's plans to "break up state education" if it gained power.
The Conservatives, Labour and the Liberal Democrats all sent their schools spokesmen to woo the conference in Birmingham. But only Ed Balls, Schools Secretary, gained a rapturous reception, with around half the delegates giving him a standing ovation.
Chris Keates, the union's general secretary, told Mr Balls that her Ofsted-style rating of his performance was "good with outstanding features" and that he was "fighting the corner for teachers".
For anyone missing the "vote Labour" message, a ballot box with a cross on it then appeared on the projection screen behind her. "I don't think anyone is in any doubt where their vote for education should be placed," Ms Keates said.
In contrast, the union's members and leaders repeatedly attacked the Conservatives' education proposals throughout the conference.
Ms Keates said the NASUWT would not battle the Tory plans single-handedly but would try to mobilise support from parents and the community - a tactic that had succeeded in its disputes with some academies.
Schools had recently been through one of the calmest periods in industrial relations for 30 years, she said, but "at some point you have to draw a line in the sand".
Shadow schools secretary Michael Gove was the first of the politicians to speak at the conference. He praised the union and its general secretary - noting the "trademark twinkle in her eye" - and promised teachers would become the "custodians and guardians of the intellectual life of the nation" under the Conservatives.
Mr Gove said teaching unions might have trepidations about a Conservative government, just as he had concerns when he switched to the Murdoch-owned newspaper The Times after being active in the National Union of Journalists. But they would be pleasantly surprised, like him, that "you can have a passionate commitment for trade union rights, then a fruitful relationship with someone seen as a right-wing bogey".
Yet he still received a few heckles. And after he suggested the NASUWT had helped inspire many of the Conservatives' education policies, Ms Keates responded that she was considering suing.
She said the party's "free school" plans would only help the "pushy and privileged" and that public opposition to this left the Conservative education policies "in tatters".
"It's freedom, in our view, to reduce the vision for 21st-century schools to children being educated in a rundown flat over an off licence," she said. "It's a corrupted definition of what freedom means - schools will be free to do what they choose as long as it's OK with you. Please stop the pretence that schools will be free or teachers will be free under a Conservative government."
David Laws, the Liberal Democrat education spokesman, fared better when he outlined his party's policies, including plans for a new type of league table which would compare schools in similar circumstances.
But he faced some light criticism from Ms Keates, who singled out his party's plan to let all schools have the freedoms of academies, warning this might damage teachers' pay and conditions.
Her greatest praise was reserved for Mr Balls. The closest she came to criticising him was to suggest he should be "bolder" in clamping down on academies and the General Teaching Council for England.
Mr Balls delivered his crowd-pleasing speech without notes, winning frequent applause and approving laughter. Between flattering the union for its gains in the social partnership, he told anecdotes about his difficulty teaching a lesson in a secondary school, and sending his children to school with NASUWT pencil cases when they had an NUT teacher.
He also announced guidance clarifying when teachers can intervene with force to stop pupils seriously disrupting lessons and school activities.
As Ms Keates had done, Mr Balls warned that the Conservative free school plans would only benefit some children.
"It is so dishonest for the Conservatives to pretend to be Santa Claus promising new schools here, pupil premiums there, without being clear that this will be paid for by cuts to school budgets and to existing teacher and teaching assistant numbers," he said.
The NASUWT earlier supported a motion calling for a campaign to defend state education, noting that at the election teachers would face "a stark choice between those who are committed to the values and ethos of public services and those who cling to the wreckage of the failed free market philosophy".
Colin Surrey, a teacher from Hertfordshire, said: "Michael Gove says he wants to raise standards - really what he wants is to bring back the Thatcher years. He is trying to use the recession to butcher our public services."