States cited the high traffic Tuesday as the reason for widespread technical glitches. Consumers have been unable to enroll in coverage, and the marketplaces' websites have displayed error messages or simply failed to load.
The glitches have ensnared exchanges run by the states as well as the federal government. New York state said its exchange received 2 million hits in its first two hours online.
Two states — California and Connecticut — had reported successful enrollments by noon, while other states had taken parts of their systems offline to hammer out technical problems. Some, including Maryland, said they expected to come back online Tuesday afternoon.
Sixteen states and Washington, D.C. are operating their own exchanges. The federal government is operating the other 36, using healthcare.gov as the primary portal for all of them.
President Obama and his administration repeatedly predicted "glitches" in setting up the new insurance exchanges, saying any technical project of this size was certain to run into logistical problems.
Republicans have said ObamaCare is "not ready for prime time" and have raised questions about both the security and basic functionality of the new insurance exchanges.