Merger negotiations between Boston law firm Bingham McCutchen LLP and the larger Morgan, Lewis & Bockius LLP are entering a critical phase.

Morgan Lewis has identified a group of roughly two dozen Bingham partners whose participation is necessary for a combination to proceed, according to two people familiar with the matter.

Such provisions are common in law-firm mergers, serving as a sort of insurance policy to reduce risk for the acquiring firm.

While those Bingham partners appeared to be on board with a deal, no definitive or signed agreement had been reached, one of the people said.

Both partnerships are expected to make a decision on whether to combine in the next several weeks, the people said.

Bingham partners who participate in a combination with Morgan Lewis would have to contribute capital to the firm, the people said.

Capital contributions, a common requirement at most law firms, though not at Bingham, are used to help fund firm operations.

Bingham declined to comment.

It wasn't clear how many lawyers and offices would be involved in a potential combination of the two firms. Tie-ups between big law firms can create client conflicts, resulting in the departure of some lawyers or entire practice groups.

Mergers also can be opportunities for firms to shed underperforming partners and eliminate redundancies, including leases.

Both firms have substantial operations in New York, Washington, San Francisco, Silicon Valley and Southern California.

A deal could expand the reach and depth of both firms in key legal markets.

Morgan Lewis, which has about 1,350 lawyers, was founded in Philadelphia and has 25 offices, including nine overseas.

Progress on a merger agreement could stabilize the partnership at Bingham, where revenue fell 13% last year. Dozens of partners have left the firm in recent months, include a defection this month that included Bingham's lucrative U.K. restructuring practice.

The firm will have fewer than 750 lawyers once those exits are completed.

Write to Jennifer Smith at jennifer.smith@wsj.com