At last month’s Republican National Convention, would-be first daughter Ivanka Trump claimed that if elected, her father would ensure that all American parents would get access to “quality” and “affordable” child care.

During a speech on economic policy on Monday, Donald Trump, in the middle of a turnaround push involving actually endorsing Republican leaders and making ostensibly grown-up policy proposals, attempted to deliver. The plan: Trump said he will allow moms and dads to deduct the average cost of child care in the United States on their annual taxes.

Whoopty doo!

Economists quickly pointed out how useless Trump’s plan would be for all too many parents.

By implementing his proposed childcare subsidy as a tax deduction rather than a tax credit, Trump effectively excludes all poor families. — Justin Wolfers (@JustinWolfers) August 8, 2016

According to Care.com, a little more than half of families are spending at least 10 percent of their annual income on child care. It’s an enormous burden. The cost is surging well in excess of the rate of inflation. The Bureau of Labor Statistics reports that the cost of child care and nursery school increased by more than 160 percent since 1990, while consumer prices during the same period went up at less than half that rate. In a majority of states, child care will cost a parent more than tuition at an in-state college.

However, there’s an enormous range of average costs, depending on where a family lives. According to the Economic Policy Institute, the monthly expense of child care for one child of preschool age can range from $344 in rural South Carolina to $1,472 a month—that’s more than $17,000 a year—in Washington, D.C.

The IRS currently offers parents a credit of $3,000 for one child aged 12 or under or $6,000 for two or more. The actual dollar amount of the tax break filers receive is determined by their adjusted gross income. This is nowhere near enough.

Deductions, like the one Trump is proposing, work on a percentage basis. The higher the tax bracket, (in other words, the wealthier the filer) the more the deduction is worth. I’ll let economist Michael Linden show you how it works:

4. For example, a family in the top tax bracket would get a $400 subsidy for every $1,000 paid in child care costs. — Michael Linden (@MichaelSLinden) August 8, 2016

5. Whereas a middle class family in the 15% bracket would get only $150 in savings for the same $1,000 in child care costs. — Michael Linden (@MichaelSLinden) August 8, 2016

And if a household is earning so little they already don’t have a tax bill? Well, yet another deduction isn’t going to do much of anything for them.

Finally, a tax deduction is unlikely to help the people who need it to pay the bills now. Taxes, after all, are filed once a year. Child care expenses, on the other hand, are paid on a regular basis. In fact, more than a few providers insist on payment in advance.

One last thing: I should mention that while Hillary Clinton is proposing a plan that would limit a family’s out-of-pocket child care costs to 10 percent of their income, there are few specifics about how it would work. But unlike Trump’s plan, it would almost certainly help the people who need the aid the most.

Read more Slate coverage of the 2016 campaign.