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No one knows how many of the currently uninsured will enroll if the court drops both the mandate and the pre-existing condition reforms. While the sickest Americans could find it impossible to buy coverage, premiums would drop and perhaps attract younger, healthier Americans.

But even if that kept enrollment numbers about the same as if the court struck down just the mandate, the fact remains that not nearly as many Americans will buy insurance if the government doesn’t require them to, analysts say.

“A lot of that coverage you’re getting is because of the premium subsidies,” Mr. Holahan said. “You’re still only getting 40 [percent] to 50 percent as many people covered.”