"Adult children aren't necessarily dependents for tax purposes, but an employer can allow anyone to be on a plan, just like they now allow domestic partners," said Fronstin. "If your employer said, 'I'm going to let you keep this,' it would become a taxable benefit for certain people."
Advocates for the elderly are also worried about untoward ripple effects.
If the entire law is overturned, seniors with high prescription costs in Medicare's "donut hole" coverage gap could lose annual discounts averaging about $600. AARP policy director David Certner says he would hope the discounts could remain in place at least through the end of this year.
Yet that might not be possible. Lacking legal authority, Medicare would have to take away the discounts. Drugmakers, now bearing the cost, could decide they want to keep offering discounts voluntarily. But then they'd risk running afoul of other federal rules that bar medical providers from offering financial inducements to Medicare recipients.
"I don't think anyone has any idea," said Certner.
A mixed verdict from the high court would be the most confusing outcome. Some parts of the law would be struck down while others lurch ahead.
That kind of result would seem to call for Congress to step in and smooth any necessary adjustments. Yet partisan divisions on Capitol Hill are so intense that hardly anyone sees a chance that would happen this year.
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