Reserve and National Guard units that get anywhere from 20 per cent to 50 per cent of their weekend warriors from among employees of the government could have serious reenlistment problems if the White House succeeds in trimming civil service military leave benefits.

Currently about 100,000 reserve and national guard members (or one of every 6 in uniform) works for either the federal, state or local government. It's been estimated that 46 per cent of the "career" guard and reserve (that is, people who have satisfied their basic military obligation and remain in service) is composed of men and women who work for some arm of government.

The Carter Administration has dusted off a plan first proposed by President Ford that would trim pay and leave benefits for civil servant-soldiers to bring them more in line with military leave policies of private industry.

Critics of the plan - and they range from federal employee unions to local commanders and some Pentagon manpower experts - warn that eliminating the military duty "subsidy" government workers now get will cause many of them to drop out of reserve and guard units when their current enlistments expire.

The federal government (and many local government bodies follow the federal practice) permit employees to take 15 days a year for annual military training without loss of vacation. Those civil servants are permitted to draw their full civilian federal salary for that specific training, in addition to their 'regular' Army, Navy, Air Force of Marine Corps military pay during that service time.

That means that a government clerk on military leave with a guard or reserve unit for summer or annual training gets his full civil service pay, plus the salary for his military rank. And no vacation time is lost.

By contrast, the majority of private firms in the country (according to a 1974 study by the Civil Service Commission) give workers lesser benefits for annual military training. Those benefits range from full pay - as the government does - in some firas to an "either or" situation where the employee gets his salary, or his military rate but not both, or company agreements where the employee is guaranteed no loss in civilian pay because of military service. In sot cases, private exployers don't give workers the same military leave benefits that automatically go to civil servants.

Pentagon budget cutters say the "overly generous" government plan permits 100,000 part-time soldiers to double-dip in pay, and that it is unfair because industry does not provide the same benefits. Some private firms, the study shows, make employees take their annual military training time as vacation or leave without pay.

The American Federation of Government union charges that the plan to cutback benefits is unfair to government workers and could hurt reserve and guard units whose strength has slipped since the draft was ended. (AFGE incidentally is studing the feasibility or organizing the military).

Pentagon officials don't want to go against the Carter plan publicly, but they are concerned that Lopping off the major leave and pay fringe benefit for government workers could hurt reserve strength. Some have quietly urged that the Administration try to Persuade private employers to follow the government example thereby making it more financially attractive for other young men and women to enter the reserves and guard. But for the moment the Carter Administration has put the cutback of civil service military leave benefits high on its federal payroll cost hit list.