Newly reunited, Exxon and Mobil might have some of the heft of Standard Oil, their forefather company that once dominated the American oil industry. But they do not have the old chutzpah. The proposed colossus lacks one thing that made Standard Oil the prime target of antitrust crusaders and muckraking journalists: a Rockefeller.

Even before President Theodore Roosevelt's antitrust campaign finally shattered Standard Oil into 34 pieces in 1911, John D. Rockefeller, the founder, had retired from management. Today, many of his 140 descendants have shareholdings in Exxon, Mobil and other cousins of the Standard Oil family, but none has significant stakes or exercises control.

''I don't think that any member of my family has been involved in the oil industry for almost a century,'' said David Rockefeller, a soft-spoken third-generation scion and a somewhat reluctant spokesman for the family's petroleum industry patrimony. ''We have all gone on to other things.''

Of Rockefeller descendants who still carry the family name, John D. Rockefeller's only son, John Jr., became a leader in philanthropy. Of John Jr.'s children -- David's generation -- two became bankers and investors, two went into politics, and one, John 3d, spent most of his life giving away money made by his grandfather.

So, if the Justice Department decides to challenge the Exxon-Mobil merger on antitrust grounds, it might find the target less tempting than Rockefeller's Standard Oil. John D. Rockefeller personified the trust kings of the Gilded Age, who accumulated unprecedented and to this day unmatched power over key industries like oil, steel and railroads. Faced with the Sherman Antitrust Act of 1890, Mr. Rockefeller was brash, removed and defiant. He was in the popular mind the plutocrat who inspired an activist Government role in regulating big business.

Ron Chernow, author of ''Titan,'' a biography of the elder Rockefeller, compares the Standard Oil founder to William H. Gates, chairman of Microsoft, which is currently the target of an antitrust lawsuit. Both created dominant companies from scratch, drove rivals out of business through aggressive pricing, and innovated relentlessly to maintain their franchises. In both cases, the Government attacked the methods of the individual businessmen as a core part of the threat posed by the companies, Mr. Chernow said.

''One of the main reasons for the breakup of Standard Oil was John D. Rockefeller,'' Mr. Chernow said. ''He was Standard Oil and Standard Oil was John D. Rockefeller. It was much easier for the Government to attack a company that had a face attached to it. Exxon and Mobil today is a faceless bureaucracy. Its leaders don't inspire the same emotion from the general public.''

The same could be said for the Rockefeller clan itself, whose modern reputation is anything but antagonistic. David Rockefeller is a good example. Now 83 years old, he is not the oldest living Rockefeller. But he has become a public leader for fellow heirs.

Mr. Rockefeller was chairman of Chase Manhattan Bank, where he worked for 35 years, and an international businessman who cultivated relations with royalty, power and wealth on every continent. He is famous for his Rolodex, which he has said contains in excess of 100,000 business cards. He has a personal secretary to manage his contacts. Mr. Rockefeller is currently at work writing memoirs of a life that has little to do with oil, though the book will recount his impressions of his grandfather, as well as provide a history of the family.

Mr. Rockefeller's father, John Jr., wound down the family's indirect role in the oil industry in the 1920's and 1930's. Many oil shares were sold to finance the construction of the huge Rockefeller Center office complex in midtown Manhattan, which consumed a great deal of the family's wealth and did not return profits in John Jr.'s lifetime, David Rockefeller said.

But David Rockefeller, through his role as one of America's leading corporate bankers as well as a continuing minority shareholder, has kept tabs on developments in both Exxon and Mobil, and he finds the rationale for the merger compelling.