Utility workers at the 64-year-old Kendall Square power plant are threatening to strike unless the owner meets their demands about staffing and safety.

Earlier this week, 27 plant workers represented by Utility Workers Union of America Local 369 overwhelmingly authorized its leaders to call a strike if they fail to make progress on issues with NRG Energy, of Princeton, N.J., and Houston. The contract is set to expire at midnight Thursday.