Clouding future projections of uninsured are tricky methods of counting them today. Even though legislation won't cover many of them, illegal immigrants are especially difficult to enumerate: Few raise their hands to be counted. Prof. Gruber estimates they make up about 13% of the uninsured today, or nearly six million people of that 45 million number.

Of the rest, some people are eligible for health insurance but don't know it and many can afford it but don't want it. About 43% of uninsured non-elderly adults have incomes greater than 2.5 times the poverty level, according to a report released Tuesday by the business-backed Employment Policies Institute.

Meanwhile, Census's state-by-state counts of the uninsured tend to be much higher than state surveys, which have their own flaws. For instance, some don't reach people without landline phones.

The national agency assumes that people who don't answer its health-care questions are much more likely to be uninsured.

But that overstates the number of those without coverage, according to Michael Davern, an assistant professor at the University of Minnesota’s public health school, who has been studying the discrepancy under a contract with the Census Bureau. To adjust for that overestimate, he recommends that Census adjust its national count of uninsured people downward by 2.5 million. The agency is still considering whether to implement that change.