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As a result, premiums for people who directly buy their own coverage would jump by 15 percent to 20 percent, the Congressional Budget Office estimates. Older, sicker people would flock to get health insurance but younger, healthier ones would hold back.

To forestall such a problem, the administration asked the court - if it finds the mandate unconstitutional - to also strike down certain consumer protections, including the requirement on insurers to cover people with pre-existing health problems. That would mitigate a damaging spike in premiums.