IN A tax-bill saga full of clarifying moments, there was one particularly eloquent expression of Republican priorities. First, Republicans refused to fund Florida Sen. Marco Rubio's plan to expand tax benefits for low-income families because he proposed paying for it by dropping the corporate tax rate to merely 21 percent rather than 20 percent. Then, in final negotiations on the bill, they adopted the 21 percent rate Mr. Rubio had sought, after all — and used the savings not to help needy families or to lessen the bill's impact on the national debt but to lower the top income tax rate for the highest wage earners.

After some last-minute theatrics, a smaller version of Mr. Rubio's plan was added to the final package. But it is telling how easy it was for Republicans to drop top income tax rates, how hard it has been for Mr. Rubio to secure even a limited victory for low-income people and how irrelevant concerns about ballooning the national debt have been for most in the GOP.

What started as a bipartisan push for smart tax-code reform mutated over the past year into an expensive, one-sided legislative power play animated by ideological misconceptions rather than a sober assessment of the country's real needs. This process is nearly complete, with the conclusion this week of negotiations among GOP lawmakers reconciling House and Senate versions into a final tax bill, set to be voted on early next week.

In a bill slanted toward the wealthy, Mr. Rubio's idea to expand the child tax credit for low-income families would make the overall tax plan a bit more progressive, diverting a little more money toward those who could really use the help. By contrast, the GOP plan to lower the top income tax rate to 37 percent would sharply and unexpectedly increase incomes at the top. As with previous iterations of the GOP plan, the overall picture is still of a tax reform that offers little benefit to many ordinary people while handing huge benefits to the rich at the expense of the country's long-term fiscal health.

A tax cut for corporations would be warranted under different circumstances — particularly if it were paid for. But many of the bill's provisions are simply unjustifiable. Obamacare's crucial individual mandate should not be repealed. Wealthy heirs do not require relief from the estate tax. High earners do not need a lower top tax rate.

Republicans claim that the bill's provisions will spur economic growth and Americans will share broadly in the resulting prosperity. In fact, the growth effects will probably be mild, and the price tag will probably be a trillion-plus dollars in new debt, according to nonpartisan projections. This debt will further burden low- and middle-income Americans in future generations as their government loses the means to invest in research, education, health care and infrastructure.

Independent analysis after independent analysis concluded that the GOP tax framework would exacerbate economic inequality and add massively to the debt — well more than the Obama stimulus Republicans attacked as an irresponsible monstrosity — for relatively little in return. The Republican response has been to ignore the experts, attack the messengers and double down on bad policy.