An Obamacare tax on health insurance plans could collect $15.5 billion in 2020 if Congress doesn’t step in to block it, the Internal Revenue Service announced Sept. 3.

Under the Affordable Care Act, a fee is levied on all fully insured health plans to help cover the cost of the law. The plans affected include individual market plans, small group and large group plans, Medicare managed care plans, Medicare Part D prescription drug plans, and Medicaid managed care plans.

But Congress has suspended the tax some years, including in 2019. The last year in which the fee was levied was...