The redoubtable director of the Office of Management and Budget David A. Stockman, confessed yesterday that he isn't always that consistent, or, as Stockman put it, "rigid," in his selling of President Reagan's program.

Especially where the president's program comes into conflict with a past presidential campaign promise. The program, Stockman made plain, comes first.

The chairman of the House Post Office and Civil Service Committee, Rep. William D. Ford (D-Mich.), brought up the matter near the windup of a hearing on the administration's drive for $5.5 billion worth of cuts in federal pay, and other programs under the committee's jurisdiction.

One of the most controversial recommendations is to do away with the present system of twice-a-year cost-of-living increases for retired federal workers and raise their benefits once a year instead. Under the proposal they would have to forgo an adjustment now scheduled for September.

The administration's wunderkind, Stockman, made it sound quite logical, pointing out that the millions of other retired citizens, who depend on Social Security benefits, receive once-a-year cost of living adjustments.

"It is hard to justify dissimilar treatment of these two comparable groups," Stockman submitted.

The only trouble was that the president had semmingly pledged otherwise, as pointed out by the Democratic Congressional Campaign Committee yesterday in a new compendioum entitled "The 1980 Campaign Promises of Ronald Reagan."

In a statement submitted to the National Association of Retired Federal Employes last October, candidate Reagan had said, in part, "Over the last 3 1/2 years of the Carter Administration, the consumer price index has increased by 43 percent. . . . I do not favor abandoning the present semiannual indexing."

(Similarly, according to the pamphlet, on Social Security Reagan had told the American Association of Retired Persons last fall: "Any reform of the Social Security system must have one overriding goal: that the benefits of those now receiving -- or looking forward to receiving -- Social Security must be protected, and that payments keep pace with the cost of living.")

At yesterday's hearing, Stockman had given added weight to the president's campaign pledges on another point, in an effort to emphasize how essential it was to keep the size of the federal work force under control. Stockman suggested that the OMB had little choice in the matter, since Reagan had made a "clear and unequivocal" commitment to that effect.

Ford, who began the hearing by stating how truly sorry he was that Stockman was no longer a member of Michigan's congressional delegation and how "each day we become sorrier," waited until the last minute to pounce.

He wondered whether the administration's position would be the same if the committee could come up with evidence of a "clear and unequivocal" commitment by Reagan to maintaining the twice-a-year cost-of-living adjustments for federal workers.

"That evidence has already been submitted, and the position remains what it is," Stockman replied without a moment's hesitation. He observed that critics have accused him of being "rigid, unbending" and too much of an ideologue. He said his stand on the cost-of-living issue should show that such talk was simply not accurate.