Harris Beach and Murtha Cullina, moving another step closer in their combination plans, said they have inked their agreement now.

The midsized law firms previously announced in May that they were planning to combine next year. On Tuesday, the firms said partners had signed and approved an operating agreement that will govern a new firm known as Harris Beach Murtha with more than 250 lawyers. The new firm will operate across 16 offices in Connecticut, Massachusetts, New Jersey, New York and Washington, D.C., the firms said, calling it a “regional legal powerhouse.”