Christine Blower, a 47-year-old language teacher, is standing for the post of general secretary of the National Union of Teachers against the current holder, Doug McAvoy. He has claimed that she "is controlled and supported by a group of extreme Left organisations" and that she has "a political, revolutionary platform that she and those who control her seek to impose on members".
Ms Blower, who works in Hammersmith and Fulham, west London, denies Mr McAvoy's allegations that she is controlled by extremist parties.
She offers the Left the best chance of gaining power in the union that it has had for years because, for the first time, all the left-wing groups have united behind her.
She said last week: "I think that industrial action is more likely if I am elected. I don't think the job of the general secretary is to spend time calling people out on strike, but it is sometimes the only way to achieve the union's policies."
Asked about her chances of success, she replied: "I am as confident as Mr McAvoy."
Mr McAvoy has held his pounds 79,000 a year job since 1989. He has often appealed to the union's ordinary members over the head of the annual policy-making conference. Each Easter it votes for strikes on everything from class size to failing schools but industrial action rarely materialises.
Ms Blower said: "I shall seek to carry out conference policies in a way which has not always been the case in the past."
At present, the union is in dispute with the Government over proposals to introduce performance related pay for teachers. This year's conference voted for the union's first one-day national strike for 30 years but members have not yet been balloted and negotiations with ministers are continuing.
Ms Blower said: "If conference votes unanimously for industrial action if the Government links appraisal to pay, it is incumbent on the general secretary to carry out conference's wishes."
Mr McAvoy recently told the union's local branches that "the Prime Minister would welcome my defeat and her election because it would damage and divide the union and weaken its power and influence".
In an advertisement last week he urged his members to vote. "Supporters of Militant, the Socialist Teachers' Alliance, the Socialist Workers Party and all the factions of the extreme left have already voted. They voted for Christine Blower."
Ms Blower, who is not a member of any political party, said that she was a member of the Campaign for a Democratic and Fighting Union which included Liberal Democrats and Labour members as well as more left-wing groups. "It is true that members of the groups he mentions are more likely to vote for me, but I am not controlled by them," she said. Mr McAvoy was not available for comment.