While Republicans and Democrats argue over solutions, all parties agree that some 30 million Americans lack medical insurance. But a new Census Bureau study indicates that the issue is more complex than that. In the last quarter of 1990, 32.7 million people indeed were without benefits. But for many, the problem may have been temporary, perhaps because of short-term unemployment. A study tracking individuals over 28 months beginning in late 1986 found that only 9 million lacked insurance for the full period. But 61 million were without coverage for at least one month.