The latest Republican tax reform framework has some positive attributes -- particularly how it offers a simpler tax code with fewer brackets. But three key missing details should raise alarms about how it would affect the tax burden placed on families.

More than eight months into the term of President Trump, Republicans still haven't released a detailed tax plan. Instead, what they released on Wednesday was yet another outline. On the surface, the plan promises tax relief to families, but without more information, it's impossible to say whether that's true. Under some assumptions, many upper middle-class families could be looking at a tax increase.

To start, here's what we do know. The plan would collapse the current seven individual brackets into three (12 percent, 25 percent, and 35 percent). That means the lowest tax rate will actually go up -- from the current 10 percent to 12 percent. To help offset this, the plan would nearly double the standard deduction -- to $24,000 from the current $12,700. However, the plan would also eliminate exemptions, which are currently $4,050 per person. A family of four, thus, would receive $16,200 in exemptions in the current system, plus the $12,700 standard deduction, for a total $28,900 reduction of taxable income. But under the Republican plan, they would only receive the $24,000 standard deduction. On the surface, this would expose nearly $5,000 in additional income to taxes. This is where the three big unknowns come in.

How much will the new child tax credit be? The outline promises that the plan "significantly increases" the child tax credit, which currently stands at $1,000. But it doesn't specify the amount of the increase, making it impossible to evaluate whether it will be high enough to offset the elimination of exemptions.

Which income groups will be eligible for the child tax credits? Under the current system, eligibility for the credits starts to phase out at $110,000 in modified adjusted gross income for married couples filing jointly. After that point, the credit goes down by $50 for every $1,000 in additional income above the threshold. What that effectively means is that families with one child cannot claim a credit after $130,000 of adjusted household income and families with two children cannot claim it after $150,000. The outline says the plan would, "increase the income levels at which the Child Tax Credit begins to phase out." But it does not specify those new phase-out levels.

What are the income cutoffs for the new tax brackets? This is a pretty big one. The plan specifies the rates of the new tax brackets (12 percent, 25 percent, 35 percent), but doesn't reveal which income groups will be subject to the new rates. So, for instance, if the 25 percent rate kicks in at $100,000 in taxable income for married couples, then somebody would be paying $12,000 on that income (12 percent x $100,000), but if it kicks in at $75,000, the tax burden would be $15,250 (12 percent x $75,000 + 25 percent x $25,000). But we can only guess where lawmakers will end up.

Without more details, it's impossible to say how middle-class families would make out. Even if we take Republicans at their word that they will significantly increase the per child tax credit, that still points to the biggest losers of this plan being upper middle-class families. That is, families that earn too much to qualify for the child tax credit (currently $150,000 for a family of four) but don't earn enough to benefit substantially from the reduction in top rates. As Henry Olsen of the Ethics and Public Policy Center notes, upper middle-class families typically itemize their deductions, and the Republican plan would remove all of them except the mortgage interest deduction and charitable deduction. To many in this income group, current itemized deductions add up to a higher amount than they would get under the increased standard deduction in the GOP plan.

There are a few ways to remedy this issue, but they all involve tradeoffs. For instance, Republicans could, as they promise, increase the income threshold for the per child tax credit. However, there are limits on this. The more families that can qualify for it, then the lower the credit will have to be to keep the plan revenue neutral. They could also raise the income levels at which married couples start to move into higher tax brackets. But again, this would mean recouping the revenue somewhere else.

It's important to remember that when President Obama wanted to end the Bush tax cuts for couples earning above $250,000 per year, Republicans fought hard to push the definition of "wealthy" to above $400,000. Politically, targeting this group is risky. For decades, upper middle-class families have been a reliable Republican constituency, and as Olsen notes, they are the most likely to abandon Trump.

In a Wednesday speech, Trump himself tried to pitch the framework in populist terms, as a big tax cut for middle-class families. But in reality, any relief the bill gives to the middle-class is limited by the fact that the bill also calls for large corporate tax rate cuts.

Details matter, but this framework points to large corporate tax cuts, modest middle-class tax cuts, and potential increases in taxes for families that earn more than the middle-class but don't quite qualify as wealthy.