Federal workers who want to get longevity pay raises worth 3 percent--or keep their jobs during RIFs--will have to leap new and higher hurdles starting in October if the Reagan administration has its way.

Unless Congress blocks it, or unions tie it up in the courts, the administration plans to implement a new "performance-based incentive system" (PBIS) later this year.

Under the system, the government's million-plus white collar workers--nearly 300,000 of them here--would be ranked each year in a report card system using five grades.

The "marks" workers get would determine whether they get longevity pay raises (which are now virtually automatic), and the amount of job security workers could count on at layoff time.

PBIS has five rating levels: outstanding; exceeds fully successful; fully successful; satisfactory, or unsatisfactory. It would standardize various rating systems now used in federal agencies, some with three grades and others with as many as six performance levels.

Under the present system federal white-collar workers get the 3 percent longevity raises (in addition to any regular general pay raise) every one, two or three years, depending on their time in grade. The Office of Personnel Management says that 99 percent of all employes who become eligible for the in-grades get them on the basis of a "satisfactory" rating.

The proposed new system--which is in a 60-day comment period from agencies, unions and interested individuals--would require employes get a "fully successful" or better rating in each of first seven steps of their grades to qualify for a longevity increase. Long-time employes in the last three steps of their grades would have to get ratings of "outstanding" or "exceeds fully successful" to get an in-grade raise.

If the new rating system is adopted, many employes--OPM estimates as many as 6 percent of the work force--who now qualify for the raises based on a "satisfactory" rating would be denied the increases.

Performance ratings would also be used during RIFs. Since the Reagan administration took office nearly 3,000 U.S. workers here, mostly those with the least seniority, have been fired for economy reasons.

Under the proposed changes, the first employes RIFed would be drawn from groups with the lowest performance ratings. Seniority would be a factor only within each particular performance rating group.

Federal employe unions have unanimously denounced the proposed changes. They maintain that the PBIS system would make it too easy for political appointees to get rid of long-time career workers during RIFs, or to punish the uncooperative by denying them longevity raises.

Rep. Patricia Schroeder (D-Colo.) has asked the White House to withdraw the proposed changes, pending congressional review. And several employe unions are planning to take the administration to court, on grounds that the proposals were made without union consulation. But unless the changes are blocked by Congress or the courts, they will be put into effect this year.