James Millstein, an investment banker at Lazard Ltd., had just gotten up at 4:30 a.m. yesterday, hours after the United Auto Workers and General Motors Corp. reached a historic tentative pact, when the phone rang, according to a person familiar with the situation.

It was his father, Ira Millstein, congratulating his son for the behind-the-scenes role he had played as an adviser to the UAW in helping the deal along.

The senior Mr. Millstein knows something about the travails of GM. A partner at the New York law firm of Weil, Gotshal & Manges, the 81- year-old lawyer was once an adviser to a group of outside directors at GM.

The senior Mr. Millstein acted as a catalyst for the directors, who began to question then-GM head Robert Stempel's leadership when the company spun into losses during an economic downturn in 1991.

The 1992 GM boardroom coup made the elder Mr. Millstein a legal legend and was a watershed for American corporations -- the end of an era when big-company CEOs ruled virtually unchallenged by directors or large shareholders.

The clash also put GM on a path of restructuring that returned it to profitability. But the changes didn't prove to be enough against strong competition, particularly from Japanese auto makers. GM's profit melted away again starting in 2005.

That set the stage for the arrival of the younger Mr. Millstein. The 52-year-old former bankruptcy lawyer has joined the UAW at the negotiating table on some of the most challenging issues the union has faced.

When Dan Sherrick, the UAW's general counsel, contacted Mr. Millstein in May 2005, Mr. Millstein's first call was to his father to note the coincidence. GM had changed too much for the elder Mr. Millstein to offer any insight into the company. But for the next two years, the younger Mr. Millstein would call his father frequently for insights into how troubled companies behave generally.

Mr. Millstein's job was to lead a Lazard team that scoured GM's books to evaluate whether a concession was warranted. The firm's recommendation to the union in 2005: GM needed a health-care deal -- two years into a four-year contract -- because it was in even worse shape than it would admit.

When union head Ron Gettelfinger asked him about GM's real financial state -- "Is it really bad?" -- Mr. Millstein responded, "No, it is worse," according to people familiar with the situation.

GM estimated that this earlier health-care deal saved it $15 billion in long-term health-care liabilities, or $1 billion in annual cash. The UAW then moved on to make a similar deal with Ford Motor Co.

The importance of Lazard, said a former UAW official, was to ease concerns that GM was overstating its difficulties. GM went on to lose $10.6 billion in 2005 and $2 billion last year.

Lazard then worked with the UAW on the October 2005 bankruptcy filing of Delphi Corp., a former GM parts operation. After nearly two years of court battles and tense negotiations, the UAW was able to negotiate with GM for large-scale buyouts and early retirements for thousands of Delphi workers. GM also covered much of the health-care and pension burden at Delphi.

In the end, GM estimated it would spend $7 billion to bail out Delphi, by far its largest U.S. supplier. Delphi expects to emerge from bankruptcy by year's end.

In this round of GM negotiations, Mr. Millstein and Lazard were instrumental in creating a fund for retirees' health care that will be run by the UAW but financed with a large contribution by GM, say UAW and management officials.

Mr. Millstein flew to Detroit for negotiations on Sept. 18, just as the UAW's Mr. Gettelfinger was telling GM he wanted to take the discussion of the fund, known by the acronym VEBA, off the table because the two sides were too far apart on funding levels.

With Mr. Millstein at the table, and the Lazard team analyzing various GM proposals on VEBA, a framework for the complex fund was reached about 1 o'clock Friday morning.

That allowed Mr. Millstein to fly home in time for Yom Kippur, even as the union leadership and GM negotiated.

Mr. Millstein declined to comment on the contract talks until after the agreement is ratified.

Next on Mr. Millstein's agenda is negotiating back home, in Larchmont, N.Y., with the firefighters on a health plan. Last spring, the village's Republican mayor, Liz Feld, reached out to Mr. Millstein.

"Jim doesn't look at negotiations as a zero-sum game," said Ms. Feld. "When I told the firefighters they would be dealing with Jim, they assumed he was GM. They were very surprised to know that he worked for the union."

(See related article: "Heard on the Street: Investor Focus Will Turn To the Car Operations, Where Picture Is Blurry" -- WSJ Sept. 27, 2007)