A new analysis of Internal Revenue Service data is receiving a lot of attention among conservatives for showing that the 2017 tax cuts reduced the income tax burden of working-class taxpayers by a higher percentage than it did for higher-income earners. But conservatives should also recognize the main reason: expanded child tax credits that most supply-siders opposed.

As the analysis, by Justin Haskins of the Heartland Institute, makes clear, taxpayers earning between $5,000 and $30,000 a year in adjusted gross income saw the biggest percentage drop in average income tax liability. People earning between $30,000 and $75,000 received the next-biggest percentage tax cut, with their 2018 income tax payments falling roughly 18 percent compared to their in 2017 tax bills. While all income groups saw an average tax cut, the size of the cut as a percentage of income declines as income increased. Those earning between $5 million and $10 million annually, for example, paid only 3.48 percent less in taxes in 2018 than they did in 2017.

That’s good news for struggling families, and a message Republicans should be proud to push. But it’s crucial to understand why that happened. The answer runs counter to some cherished supply-side dogma.

The 2017 tax bill helped lower-earning taxpayers with three crucial provisions. First, it replaced the old personal exemption, which eliminated income taxes on $4,150 for each person in a household, with an increased standard deduction. Second, it reduced the marginal tax rate applied to taxable income between $19,051 and $77,400 for a married couple from 15 percent to 12 percent. Third, it doubled the tax credit for each 17-year-old child or younger from $1,000 to $2,000, and added a new $500 tax credit for every dependent who did not qualify for the expanded child tax credit. This provision also made the first $1,400 of each child’s tax credit refundable, which means taxpayers would receive up to that amount in excess of any income taxes owed.

It’s easy to see why the expanded child tax credit is the primary driver in cutting taxes for lower earners. Consider a family of three earning $35,000 a year. Because of the elimination of the personal exemption, this family would have seen the amount of its income exposed to federal taxes increase by $12,450. The increase in the standard deduction from $12,700 to $24,000 would have offset most of this, but the family still would have seen $1,150 more of its income taxed than it would have before the GOP bill. With a tax rate of 10 percent, that means the family would face an initial tax hike of $115.

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In other words, the 10 percent rate, which supply-siders love to tout, would not have helped this family. The higher child tax credit, however, was a godsend to families such as this. Tax credits reduce a person’s tax liability by a full dollar for each dollar of credit. Increasing the credit by $1,000, then, reduced this family’s tax bill by the same amount. That’s a big savings.

This huge benefit faced vociferous opposition from the GOP’s supply-side wing, which tends to dominate the party’s tax policy. The expanded child tax credit was not included in the version originally introduced in the GOP-controlled House. Sen. Marco Rubio (R-Fla.) championed the larger credit in the Senate, but luminaries such as the influential Wall Street Journal editorial board opposed him. It took weeks of negotiating — and a threat from Rubio that he might not support the bill without a larger and more refundable credit — before party leaders finally gave way.

The fact that so many conservatives are now touting the net tax cut for the working class should lead to a party-wide rethinking on tax policy. Virginia’s recently elected GOP governor, Glenn Youngkin, got the message. His tax plan eschewed the orthodox rate cuts pushed by the losing 2017 Republican nominee, Ed Gillespie, in favor of increasing the standard deduction and eliminating the state’s sales tax on food.

Austria’s conservatives similarly cut income taxes for workers in the bottom three tax brackets, affecting people making less than 60,000 euros a year. They know what the GOP’s supply-side priesthood doesn’t: It’s better to reduce taxes for people who are struggling than for people who aren’t.

It’s good that conservatives promote the benefit of tax cuts for working families. They’ll benefit even more if they make that talking point into a new tax policy dogma.