Even before the Commerce Dept. issued its 1990 census data in April, northern cities and states and minority groups had sued, charging that millions of people were missed by the original head count. The stakes were high since the census data affect everything from federal aid to the boundaries of congressional districts and, thus, political fortunes. But as a court-imposed July 15 deadline approaches for Commerce to decide whether to make an adjustment, the steam seems to have gone out of the issue. Though Commerce is expected to reject an adjustment, the response from such states as Michigan, Illinois, and New York, which blasted the undercount, may be muted. They now realize they wouldn't gain anywhere near as much from a revision as they originally thought.