If you believe that the union is discriminating against stewards based on who won't or will support the union agenda (and is against free thinkers) you have two primary options.
You can circulate a petition that you want a vote on the current training and certification process for union stewards, to give more power to bargaining units in ensuring that they have appropriate representation in the union. You could petition that there be proportional representation for the bargaining units to ensure that each one has fair representation and prevent them from blocking the certification of your bargaining unit members.
The other option is to file an unfair labor practices complaint against the union with the National Labor Relations Board that they are infringing on your bargaining unit's member's rights under the National Labor Relations Act in preventing the certification of stewards from particular bargaining units, and in turn, not providing fair and equal representation to all union members.
If you are interested in pursuing the second option, you can file an investigative charge with the National Labor Relations Board by contacting your regional office of the National Labor Relations Board here:http://www.nlrb.gov/who-we-are/regional-offices
Although this may be an unpleasent path to take, it may be necessary one in order to ensure that their is fair and proportionate representation and that there are at least some stewards who can provide dissenting opinions to the union's current direction. (Even if the NLRB doesn't prosecute the unfair labor practices, the beginning of an investigation could prompt the union leadership to take proactive measures to ensure they are not investigated or charged in the future, including allowing members from your bargaining unit to be certified as stewards again).
I hope the above information is helpful.
Please let me know if you have any follow up questions.
Thanks and best of luck!