EVER SINCE Richard Nixon vetoed the Comprehensive Child Development Act of 1971, day care had been off the agenda for public discussion in the United States. But now, through one of those processes that nobody really understands, the issue has resurfaced. The economic realities facing American families turned out to be more powerful than Norman Rockwell images and antifeminist backlashes. It is not dear exactly what kind of day-care proposals will emerge from the Bush administration, but that some kind will seems almost beyond dispute.