For months now, the Federal Reserve has been poised to raise interest rates, but has held off. Why?

The answer lies mainly in what the Fed's chairman, Alan Greenspan, describes as a powerful recent force in the American economy: job insecurity. In his testimony to Congress yesterday, he clearly elevated this insecurity to major status in central bank policy. Workers have been too worried about keeping their jobs to push for higher wages, he said, and this has been sufficient to hold down inflation without the added restraint of higher interest rates.

For more than a year now, Mr. Greenspan has been gradually building his case that downsizing and job insecurity have altered the normal workings of the American economy, and yesterday he spelled out this view more pointedly and in more detail than before. Other factors have contributed to ''the softness in compensation growth'' despite a low unemployment rate, he said, but ''I would be surprised if they were nearly as important as job insecurity.''

Job insecurity, in Mr. Greenspan's hands, is a rather complex monetary indicator. That workers are fearful of losing their jobs is not in itself a sufficient explanation for their failure to push for significant wage increases. The sense of job insecurity has to be rising; that is, getting worse. Once it levels off, and workers become accustomed to their new level of uncertainty, their confidence may revive and the upward pressure on wages resume. That is particularly true when the unemployment rate is quite low, as it is today, suggesting labor shortages that will foster greater competition for skilled workers. Inflation eventually rises as employers raise prices to pay for the higher labor costs.

Job insecurity is notoriously difficult to measure and a lot of people think Mr. Greenspan may be tilting at windmills. Some labor economists, like Henry Farber at Princeton University, say, ''There is no direct evidence that job insecurity is higher than it was.'' Many traders in the bond market, who always seem braced for the next outburst of inflation, are impatient with the Fed's focus on job insecurity. ''The whole concept has never sat well with Wall Street,'' said David Jones, chief economist at Aubrey G. Lanston. ''It just seemed like a rationalization to explain why wages had not yet accelerated in response to full employment.''