Unions have the right to bargain whenever there is a change in wages, hours or working conditions. In Worcester, the president of the police supervisors’ union — which is still in negotiations with the city — said that while body cameras would increase public trust, they had also brought about “the most significant changes to the department in decades.”

So far, state labor boards have agreed with union arguments that cities cannot impose body cameras without negotiating at least some aspects of their use, according to William Aitchison, a lawyer who represents unions and has written several books on public safety labor issues.

Those rulings have given unions a say in policies like when the devices must be activated and how often supervisors are allowed to review footage. Perhaps most controversially, many unions have won the right for officers involved in a serious incident to review body camera video before giving a statement to investigators. In Worcester, officers may view the footage only after writing their initial report, but before giving additional statements.

Years of negotiations in Portland, Ore., which has been under federal oversight for civil rights violations, yielded this unusual compromise: In cases of deadly force or serious injury, officers cannot view body camera footage before being questioned — but neither can the investigators who are going to question them.

More and more, unions have used their bargaining right to demand more pay. Cities, many of which are struggling to fill vacancies on their police departments, may be willing to concede, using body cameras as a way to increase compensation for police officers without having to give similar raises to other public sector workers, Mr. Aitchison said.