The historical behavior of unions is commensurate with the historical behavior of employers and strike-breakers.

Your adversary is not going to be ethical and nonviolent.

Unions resolve the prisoner's dilemma in favor of the prisoners. The game is set up like this:

  In each trial, 3 players distribute $300.
  A and B vote on whether E gets $100 or $150.
  E can cast a tie-breaking vote.
  E decides how to distribute the remainder to A and B.

  | A gets | B gets | E gets |
  +--------+--------+--------+-----
  |   $100 |   $100 |   $100 | A $100, B $100
  |   $150 |   $  0 |   $150 | A $150, B $100
  |   $  0 |   $150 |   $150 | A $100, B $150
  |   $ 75 |   $ 75 |   $150 | A $150, B $150

Obviously, if A and B cooperate, and voluntarily form a cartel with the power to enforce cooperation, they will experience a better outcome. Because in the long run, in repeated trials, the employer will be pocketing an extra $50 in a huge number of trials, while pitting the employees against each other.