Hank Roberts is unhappy about rules that will make it "extraordinarily difficult" for him to stand in the election for general secretary of the National Association of Schoolmasters Union of Women Teachers.
A veteran campaigner for a single, merged classroom teaching union, he must secure 25 local branch nominations in just over three weeks to stand against Chris Keates, acting general secretary.
If no one reaches the total by October 18 then Ms Keates, who won a unanimous nomination from the NASUWT's executive committee last Friday, will take up her post permanently without an election.
Mr Roberts said: "The aim of elections is to stop self-perpetuating cliques running unions, or anything else.
"Elections are a good thing and hurdles to get them should not be placed so high as to prevent ordinary members having any chance."
The 25-nomination rule was introduced by the NASUWT in 1996 after Brian Williams, a pro-unity candidate from Cardiff, challenged Nigel de Gruchy for the leadership in 1989. He only needed one nomination and won around a third of the vote.
Jerry Bartlett, the NASUWT official administering the leadership selection process, said that Mr Roberts was an experienced trade unionist who was aware of the rules and could have objected when they were introduced.
"I would say that anybody who cannot get nominations from 25 of 365 branches would have very little chance of being elected," he said.
The NASUWT executive committee received 10 expressions of interest and two formal applications for the general secretary post. One was from Ms Keates, the only shortlisted candidate, and the other from an unknown, unsuccessful applicant.
Mr Roberts last year came within 104 votes of beating executive nominee Mary Bousted in the Association of Teachers and Lecturers' general secretary election. He did not apply to the NASUWT executive committee as he felt he did not stand any chance.
This week he won his first NASUWT nomination from the Brent branch in North-west London, but he believes it will be virtually impossible to secure another 24 in time. If he does, a postal ballot of members will run from October 25 to December 3.
The post, which pays around pound;86,000 annually, became vacant after Eamonn O'Kane died of cancer in May.