San Francisco-based firm Thelen Marrin Johnson Bridges and Reid & Priest in New York are poised to merge.
The merger would create a top 50 national firm with around 350 partners 190 from Thelen Marrin and 160 from Reid & Priest.
As well as a strong presence in New York and San Francisco the firm would also boast at least 50 lawyers in both Los Angeles and Washington and a sizeable office in San Jose servicing Silicon Valley.
The firms have been in talks for a year, but Reid & Priest chairman Thomas Igoe said a merger was "three to four weeks away".
He said there was a good cultural fit between the firms as they were around the same age 60 years and the partners got on with each other.
Richard Gary, chairman of Thelen Marrin, said the planned merger with give the firms a good geographical spread across the US.
"There are few practices in the US with that broad a geographical range of capability, with lawyers in the four most important legal centres."
However, he was more cautious about the timing of a tie-up. "Three to four weeks is an overstatement. Talks are serious and substantive but the proposal hasn't yet been put to either partnership."
There are good synergies between the two firms. Reid & Priest's public utility and financial focus complements Thelen Marrin's two-third bias toward litigation and its strength in construction and engineering.
Although the new firm would have no overseas offices, the combined project finance groups would increase the firm's muscle on the international stage.
Gary would not comment on any future plans for a London office.
Last week, The Lawyer reported that merger talks between a second set of firms in San Francisco and New York Orrick Herrington & Sutcliffe and Donovan Leisure Newton & Irvine were in doubt over objections from a mutual client.