"Either insurers will drop out or insurers will raise premiums," Larry Leavitt of the Kaiser Family Foundation told The Hill.

Insurers and many analysts believe the companies won't be able to turn a profit unless young, healthy people enroll in the plans to offset the costs of the predominantly older population that has enrolled so far.

To that end, the Obama administration has raised the fine imposed on those who go without insurance. The average fine will be $695 this year for an individual adult.

Insurers have also accused the Obama administration of being too lax with enrollment rules, saying that "special enrollment periods" that allowed people to sign up for plans outside of the open enrollment period were allowing people to wait until they needed care to join plans. In response, the administration has done away with certain exceptions to the open enrollment deadline and has vowed to more aggressively scrutinize special enrollment applications. 

The administration insists that the marketplace remains stable. Mandy Cohen, chief operating officer of the Centers for Medicare and Medicaid Services, told The Hill that there was "absolutely not" a risk of the system collapsing. The millions of new enrollees in the past year portend success for the marketplace, she argued.

"What brings us the most confidence about the long term stability and health of the marketplace is its growth," she said.