“When we first saw the numbers, everyone’s eyes kind of bugged out,” said Matt Salo, who runs the National Association of Medicaid Directors. “Of the people walking through the door, 90 percent are on Medicaid. We’re thinking, what planet is this happening on?”

The yawning gap between public and private enrollment is handing Republicans yet another line of criticism against President Obama’s health overhaul — that the law is primarily becoming an expansion of a costly entitlement program.

Supporters, however, caution against reading too much into the early numbers. Some of the states that set up their own exchanges, including Maryland, are suffering Web site glitches similar to those of the national system, and that is delaying private plan enrollments.

But if this trend continues, experts say it could prove costly for states that will have to help pay for some of these new Medicaid enrollees. It would widen disparities between the states that opted to expand the entitlement program and those that have not.