Bert Lance swept into town with important credentials. He not only was as close to Jimmy Carter personally as any of the Georgia "Mafia," but represented the same conservative fiscal instincts.

"I think I understand him probably as well as anybody in the country, with the exception of Rosalynn," the big, easy-going Lance told a group of editors just before taking over as director of the Office of Management and Budget.

Lance made clear, and the White House never denied it, that he has been one of the very few who have earned the privilege of saying "no" to Jimmy Carter.

Since the OMB job's strength is derived from the degree of presidential rapport and confidence. Lance is regarded by many people as the second most important man in the Carter administration. In making up the budget, it is the function of the budget director to say "no" to Cabinet officers pushing their own programs. And Lance has had the authority to make his "no's" stick.

But he has been more than the President's "no" man. As Carter's close friend who reinforced the President's own conservative fiscal instincts, Lance became the presidential ambassador to a still-suspicious business community. It was Lance who publicized the administration's commitment to balance the budget, reorganize the government, and make effective the idea of "zero-base budgeting."

"Jimmy may campaign liberal," Lance said, "but he governs conservative."

Lance was acceptd by the business community as its principal point of contact with the administration. Busnessmen found to their displeasure that Carter, unlike former President Ford, would not make himself personally available except to a few old Georgia acquaintances. But there was the comforting knowledge, at least, that Lance could speak with definitive authority on Carter's behalf.

The former Georgia banker played the key role in limiting the scope of the original economic stimulus program proposed by Carter, and then in persuading Carter to drop the $50 tax rebate, primarily because the business community feared its inflationary impact.

Lance brought with him, as well, the businessman's more or less standard fear of "the encroachment and interference" of government in private affairs. In that role, he successfully argued against suggestions made by Council of Economic Advisers Chairman Charles L. Schultze about voluntary anti-inflationary wage-price guideposts.

On the other hand, when it served the President-elect's purpose of the moment last December, Lance didn't hesitate to engage in a little "jawboning" with Edgar Speer, chairman of U.S. Steel, and Lewis Foy, chairman of Bethlehem Steel, after they raised prices.

Said another of the Georgia "mafia" in an interview a few weeks ago: "His responsibilities go far beyond the budget to include a broad range of personal, political, and fiscal advice."

If Lance ultimately resigns, there is no one else in view to fill ths role of special confidant to the President. It can only be assumed that others of the Georiga "Mafia," especially White House assistants Hamilton Jordan, Stuart Eizenstat and Jody Powell, would each, in his own way, fill the gap.

On the economic advisory team, Treasury Secretary W. Michael Blumenthal would remain the administration's main salesman for such things as tax revision and international finances. Blumenthal is highly regarded by the financial community, but they know he doesn't have the Lance rapport with Carter.

Neither does Schultze, who, with Lance and Blumenthal makes up the "Big Three" of presidential economic advisers. He has built up a strong relationship with Carter, but it's not a casual, tennis-buddy tie like Lance's. And neither Blumenthal nor Schultze seems equipped to function as a personal political emissary to Capitol Hill.