THERE IS A LARGE acknowledgment of failure in President Carter's decision to raise federal pay 7 percent instead of 5.5 percent next month. The failure is, of course, the administration's inability to bring inflation down. When 7 percent was still a reasonable guideline for wage and price hikes in the private sector, holding federal raises to 5.5 percent made sense as a good example and a slightly deflationary step. Now, with inflation rocketing ahead, keeping pace with the private sector -- as measured by comparability formulas -- would require raises of about 10.4 percent. Thus Mr. Carter has been pushed to a higher middle ground between that form of indexing and stinginess.

Assuming that Congress goes along, which seems likely, this will be the second straight year that general federal increases have been held down.The cumulative loss of buying power is going to cause some pain, especially for workers in lower brackets. Yes, federal employment does offer special compensations, such as job security and excellent fringe benefits. But those are not much solace at the grocery store. Unfortunately, the current pay-adjustment law makes it hard to give larger increases to those in lower-paid jobs or higher-cost areas. This is one more reason for overhauling the salary system, as the administration has proposed.

Congress is apparently not about to tackle the whole federal pay system this year. But there is one aspect that the lawmakers will have to take up, however reluctantly, this month. That is the always-prickly problem of their own pay and the top-level executive salaries that are tied to it. Those salaries have been frozen since February 1977. If Congress does nothing, the annual-adjustment system will provide two raises in October -- this year's 7 percent, plus the 5.5 percent increase that was deferred a year ago. That would be too much. So far, though, the legislators have been unable to agree on a more seemly figure. When the House tried in June, it would up in a general snarl and the subject was shelved.

How much should Congress give itself? There is no magic point at which fairness and political comfort balance out. The more important question is whether Congress will be able to look beyond its own pay problems and grant some relief for the thousands of federal executives whose salaries are being held down. Many of them have had no increase for 2 1/2 years. Moreover, each general raise in the federal scales pushes more people against the ceiling, causing more inequities. At a time of so much emphasis on improving federal management, Congress should treat the top federal managers more reasonably.