A large-powerful labor union like the United Auto Workers has a number of measures it can take in response to a seemingly impassable roadblock to negotiating an agreement with management. It can authorize its members to conduct a work stoppage or a strike, while establishing picket lines just outside company property. It can instigate a work slowdown, in which the efforts of each employee on an automobile assembly line deliberately reduces his or her level of productivity to the minimum necessary to keep the line moving. It can impose the strictest interpretation of already agreed-upon principles such that management grows increasingly frustrated with the bureaucratic measures such an action would entail, or it could encourage all employees to begin taking all available sick, personal and vacation days so that a work slowdown is in effect without the across-the-board worker slowdown involved in that measure.
The most draconian measure, of course, involves a strike, in which all union employees walk off the job and establish picket lines across all entrances to the property. Management is then left with the choice of either shutting down the factory, or attempting to hire nonunion laborers. Nonunion laborers who choose to cross the picket line are derisively referred to by union workers as "scabs," and making the decision to enter and exist a workplace everyday while being subjected to hostile taunts, and possibly worse, requires a serious commitment on the part of such otherwise unemployed individuals.
The most obvious downside to initiating a strike involves the total loss of income that the striking workers would experience. Unions maintain financial resources for the purpose of aiding striking workers, but there will be financial penalties even with such assistance, and the longer the strike continues, the more the financial hardships will be on those workers.
In addition to the above measures, union headquarters could consider filing a lawsuit against the company in question for whatever alleged breach of contractual agreements may be involved, or for alleged "unfair labor practices." For example, in Oakland, California, the union representing workers at the Bay Area Rapid Transit (BART), which operates the city's commuter subway system, filed suit against the company for its alleged failure to negotiate in good faith on the issue of worker safety. In Lorain, Ohio, a labor union representing construction workers recently filed suit against the city's school for alleged failure on the part of the city to abide by its (the union's) interpretation the contruction contract for new schools scheduled to be built. Such law suits are fairly common, but can be expensive for the union if the case drags on interminably.
These, then, are the basic measures available to the UAW in any hypothetical labor dispute with the owners of the companies for which its members work.