Q: Suppose that you had a group of about 25% of retail workers in a store (i.e., enough to make an impact on things, but not enough, perhaps, to get the NLRB election process going). Suppose as well, though, that you had similar sized groups in a number of stores, spread out throughout a retail chain. You don't really have a majority in any one store, but you may have a large minority in a lot of them. Could you still act as a union? How?
A: This concept is commonly referred to as 'minority,' 'solidarity' or 'Open Source' unionism. The idea actually has quite a long history. It's also one that people have been talking about a lot again, lately. Minority unionism could be a solution to some of the problems unions have been having with organizing unions under the time-consuming and management-friendly NLRB processes. Minority unionism is, however, legally tricky, at least when it comes to actually negotiating a contract.
The law
When a union represents the majority of workers in a bargaining unit, management has a 'duty to bargain in good faith' with that union. How that union proves majority support, however, is itself open to negotiation. For example, the company and the union can agree to a card-check process where the union collects authorization cards and presents them to management, and when the union presents enough cards to show that 50% +1 members of the bargaining unit support it, then the company and the union sit down to negotiate a contract. Such a process, however, requires a company that is either exceptionally enlightened, or sufficiently cowed by the union's power that it would just be a waste of its time and money to bother putting up a fight over recognition.
If the company decides to put up a fight, though, and the union decides that it wants the government to force recognition on the company, then the route the parties have to go is generally to an NLRB election.
In between card-check and NLRB elections there are some other options, such as having an independent third party like the American Arbitration Association or the League of Women Voters run an election, with different rules, and with each party agreeing to respect the results. But these options also must be negotiated between the union and management, and generally depend on the same mix of 'enlightenment' and 'coercion' that apply to the card-check.
The minority union issue comes up when, like in the example above, perhaps only 25% of workers want a union. Here, not only does management have no duty to negotiate, it would actually be illegal for them to sign a collective agreement with such a group.
On the other hand, that 25% still do have a basic legal right under the National Labor Relations Act 'to engage in concerted activities for mutual aid and protection.' This pretty much covers any activity that workers do together in order to maintain or improve their working conditions. It may even include an action by one single employee, if the purpose of that employee's action was to inspire the rest of the group to act, or even just to represent their common interest.
For example, if management fires a popular employee, that 25% could walk off the job to demand that employee's reinstatement and (theoretically) shouldn't be fired for their actions.
The practical reality
The above outlines some of the legal issues. Practically, however, this whole thing is a power game. The labor laws are weakly enforced. Even with an NLRB-certified majority union, the 'Duty to Bargain in Good Faith' doesn't mean a heck of a lot, as Ann Arbor and Minneapolis have found out. All that management had to do in order to fulfil their duty was show up at a few meetings, and not that many meetings at that.
As for when you don't have a majority union, but rather about 25% - While the 25% are ostensibly protected, management can usually get around the law to get rid of them if it really wants to. At the very least, if management fires them illegally, it will take so long for them to get their jobs back through the legal processes that most of them probably won't want the job anymore, anyway.
What will prevent management from acting this way is if that 25% can make it sufficiently more painful to fire them than it is for management to compromise with them. If that 25% has that leverage, even though they can't legally negotiate a collective agreement, they can still de facto achieve many of the same results. Further, a powerful 25% like that could also convince management to agree to some kind of card-check recognition arrangement. And if they can do so, so that management gives up many of it's legal delay and pressure tactics against unionization, then it is a lot easier to get to that 50% +1 and then negotiate a full contract after that.
So then the question is how do you organize that 'powerful' One-in-Four?
Further reading at The Nation magazine.