Businesses are pushing back on the Internal Revenue Service’s decision to begin enforcing the Affordable Care Act’s employer insurance mandate, challenging penalties that run into the millions and asserting the agency is wrong to impose the fines.

The ACA imposes a penalty on employers with more than 50 workers who don’t provide qualifying coverage to employees, but the fines weren’t initially enforced. In November, the IRS said it would begin assessing penalties, starting with companies that failed to comply in 2015, when parts of the employer mandate first kicked in.

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