CBO and [the Joint Committee on Taxation] expect that premiums for health insurance will tend to increase more rapidly than the threshold for determining liability for the high-premium excise tax, so the tax will affect an increasing share of coverage offered through employers and thus generate rising revenues. In response, many employers are expected to avoid the tax by holding premiums below the threshold, but the resulting shift in compensation from nontaxable insurance benefits to taxable wages and salaries would subject an increasing share of employees’ compensation to taxes. Those trends in exchange subsidies and in revenues related to the high-premium excise tax will continue beyond 2025, CBO and JCT anticipate, causing the net costs of the ACA’s coverage provisions to decline in subsequent years.

In other words, under current projections the tax will grow so quickly that it will exceed the annual rising costs of the law’s new entitlements, causing net spending on Obamacare actually to decline.

The Cadillac tax has always caused the administration political heartburn. In 2008, then-Sen. Barack Obama broadcast the most-aired political ad in a decade, attacking Sen. John McCain for wanting to tax health benefits. The Cadillac tax technically targets insurers, not individuals, but videos of remarks by MIT economist Jonathan Gruber, who advised the administration when the health-care law was being developed, show Mr. Gruber saying that Democrats engaged in semantics about the tax and even “mislabeling” to provide political cover for the president to change his position.

When Obamacare was passed, Mr. Gruber wrote articles—promoted at the time by the administration—saying that the Cadillac tax wasn’t a tax. He argued that, in response to the law’s pressures, firms would reduce their health benefits but increase taxable wages—and that paying taxes on these higher wages amounted to a net plus for individuals rather than a tax increase. But in the face of pressure from labor unions, which remain opposed to the tax, Democrats ultimately decided to delay its implementation until 2018, after President Obama leaves office.

In its analysis last week, CBO made clear that the Cadillac tax, coupled with provisions slowing the growth of insurance exchange subsidies (provisions that some liberal groups want to overturn) is central to making the law fiscally sustainable. The question is whether the effects of the Cadillac tax would be any more politically sustainable in 2018 and beyond than they were in 2009—and what supporters of the law will do if they aren’t.

This post was originally published at the Wall Street Journal Think Tank blog.