You are going to have to show some evidence for that. Really the only guarantee is that the union will negotiate for what the most influential union member wants, not even necessarily the majority.
That's still up to the majority -- that is, up to the members to ensure that it doesn't happen. If they're not diligent and vigilant then that can happen.
But not if they are actually on the watch.
Democracy (in a country or a union or whatever) is a participatory sport, it's not about voting every 4 years and watching from the sidelines for the rest and/or voting scoundrels again and again.
You are going to have to show some evidence for that. Really the only guarantee is that the union will negotiate for what the most influential union member wants, not even necessarily the majority.