Midlevel federal workers who traditionally move into most of the bureaucracy's top-paid career civil service jobs will lose much of their in-house edge next month.

Uncle Sam, in the name of civil service reform, is taking steps to de-emphazise the decades-old reliance on promoting from within government to get people for its coveted "supergrade" cadre at grades 16 through 18.

Beginning in July, most of those supergrade slots will become part of the new Senior Executive Service. The SES promises higher pay faster promotions and more varied assignments. People going into it will trade tenure for the chance at five-figure bonuses, sabbaticals and more generous retirement.

SES backers - President Carter is the major one - pledge that the new system will not be more political. However, they are taking steps to ensure that more outsiders are brought into the government at top career ranks. Where you stand on that proposition depends on where you sit . . .

Example: If You are inside government at midcareer, the prospect of changing the rules to zip some whiz kid from Xeroc, GM or the Fuller Brush Co. into a senior federal job is disquieting. If you are outside government, or suspicious of homegrown bureaucratic products, the idea of more new blood and outside talent coming into government sounds good.

Within the next few weeks, the government will "inactivate" the files of nearly 27,000 grade 15 workers who form a talent pool kept as part of a computerized executive inventory. That system, set up with much fanfare some years back, once gave agencies primary leads to people with supergrade qualifications.

Better than half the total number of grade 15 employes in government live in metro Washington. Many of them have counted on their head-quaters service (and connections), their longevity and the up-from-the-ranks tradition to boost them into supergrade jobs. But the new plan, under the Civil Service Reform Act, is to broaden the recruiting base, and bring in many more outsiders to fill career executive jobs.