Two years after it was reinstituted, registration for a military draft in the United States is facing serious practical and legal challenges.
Despite Uncle Sam's crackdown on draft dodgers, large numbers of young men still are refusing to register.
The General Accounting Office (GAO) reports this week that some 700,000 have failed to report to the Selective Service System. Among those who have registered, according to the GAO, several times this number have not kept the federal government up to date on their whereabouts, an equally serious offense.
During April, May, and June of this year, registration compliance dropped to 78 percent, despite the end of the ''grace period'' offered by President Reagan and the beginning of prosecutions.
This is significantly below the compliance rate during the Vietnam war years and much less than the 98 percent figure cited by the Selective Service's director, Gen. Thomas K. Turnage as necessary for the system to be ''fair.''
In House testimony July 28, General Turnage chose to emphasize the positive, pointing out that 93 percent of those required to register (since registration was reinstituted in July 1980) so far have done so. This totals 8.4 million young men. But Turnage added that any amount of noncompliance ''is a matter of great concern.''
In August, Selective Service officials will begin matching their files with social security numbers. Non-registrants will be notified, and federal officials expect this to result in a better compliance rate.
But this no doubt will also result in legal battles over possible invasion of privacy, as civil libertarians and draft opponents have promised.
Other legal questions are being raised over conscientious objectors who refuse to register on moral or religious grounds.
Under the current system, such men still must register since they cannot apply for ''CO'' status until they are actually drafted. Draft opponents say this violates earlier law and practice that allowed COs to claim such status as soon as they registered.
''The system is riddled with constitutional and technical errors which should tie up cases in the courts for years, if not cause the whole system to be voided in the meantime,'' Barry Lynn, president of Draft Action, told the congressional panel.
The Justice Department has estimated that it will cost $25,000 to successfully prosecute each antidraft registration case.
The Justice Department has referred 159 cases of noncompliance to federal prosecutors, and five men have been indicted so far.
The GAO has suggested that more men might register if one brief period were designated and publicized each year, rather than having each young man be responsible for registering when he turns 18.
In Congress, meanwhile, there is a move to withhold any federal funds (including student loans) from those who fail to register for the draft. But Education Secretary Terrel Bell reportedly says this ''would require a level of unnecessary federal intrusion and administrative complexity and burden which we roundly oppose.''