Trade unions versus Shareholder unions

If a person owns shares in at least one for-profit corporation, and the person opposes on principle the notion that people who have gotten into positions of authority in the corporation should be able to use that authority to spend corporate money to not merely influence elections in which shareholders vote on matters involving the governance of that one corporation, but to influence the outcome of political elections for the government of a county, town, state, or country, then what should the person do?

Selling the shares isn't very useful, because there's no reason that the buyer would have any more influence than you have. Also, it is perhaps analogous to thinking that problems throughout the world with the behavior of governments could be solved if everybody in the world tried to migrate from one country to another. That's unlikely to be an effective approach.

How about creating or joining a union of shareholders? The goal of the union would be to persuade all shareholders to join the union and to use their status as shareholders to change company policy so that every shareholder gets a choice: get your share of the advertising money in the form of dividends, or allow your share of the advertising money to be used to influence political elections of the broader society.

Can you think of any grounds for opposing such a policy change? I can think of only one: an alleged free rider problem. The hypothesis is that for-profit corporations know better than shareholders how the general public should vote, and that for-profit corporations use that know-how for the good of society, so it would be unfair for some shareholders to get money when other shareholders are making a financial sacrifice to ensure that for-profit corporations use their wisdom to influence the outcome of elections in broader society.

Can you think of any other grounds for opposing such a change to the rules of how a corporation operates?

Anybody who owns shares in corporation XYZ could picket the headquarters of corporation XYZ, just as trade unions picket. Where two or three gather, a shareholder union can begin its existence.