Today, Jeb Bush became the fourth GOP presidential candidate — after Marco Rubio, Rand Paul, and Chris Christie — to unveil a detailed tax reform proposal. It's the most tempered plan to date, mostly including cuts that the Republican establishment has backed for years, like eliminating the estate tax. But it also serves as a compromise between two competing factions in the Republican Party who've been fighting for years about how best to cut taxes.

The basics: taxes for workers and families

Under the plan, there would be three individual tax brackets: 10 percent, 25 percent, and 28 percent. Compared with the current top rate of 39.6 percent, that's a drop of nearly a third.

Bush would "nearly double" the standard deduction and expand the earned income tax credit; Bush claims 15 million more Americans wouldn't have to pay income taxes under this plan.

Bush would eliminate employer-side payroll taxes on workers over 67 as part of his plan to boost economic growth by having people work more.

Like almost every GOP tax plan, Bush's would eliminate the estate tax and the alternative minimum tax; it also claims to "eliminate the marriage penalty," though it's unclear how, and removing the tax penalty for dual-earner households is easier said than done.

The plan would cap deductions other than the charitable deduction for high earners. Bush has not yet specified how the cap would work; President Obama has proposed something similar.

Bush would eliminate the "carried-interest" loophole under which hedge fund and private equity managers pay lower, capital gains tax rates on income they get from their jobs. This is a reversal from past Republican candidates, who've mostly defended the loophole as legitimate.

The basics: taxes on corporations

Bush would cut the corporate tax rate — currently at 35 percent for companies with profits over $18.3 million — to 20 percent.

Bush would adopt "territorial" taxation, where US companies don't pay taxes here on profits made overseas.

He'd adopt a "repatriation holiday," where companies could bring profits parked overseas back to the US and only pay an 8.75 percent tax (which they could pay over 10 years), rather than the normal 35 percent tax.

He'd let companies immediately deduct investments. Currently, if a company buys equipment, it must deduct the equipment's cost as it's used, over a number of years, rather than claiming the whole deduction in the year of purchase. So office furniture, for example, must be depreciated over seven years. Bush would end that.

How much would it cost? Who would it help?

As of Tuesday night, the plan is much too vague to be scored by the Congressional Budget Office, or even a think tank like the Tax Policy Center, so it's hard to estimate its budgetary impact. That said, slashing individual and corporate tax rates; eliminating the estate tax, alternative minimum tax, and taxation of foreign business income; increasing the standard deduction; and making it easier for businesses to deduct investments are all changes that'll cost money.

Bush claims, "Taken together, these policies will unleash increased investment, higher wages and sustained 4% economic growth, while reducing the deficit." This is difficult to believe. It's hard to imagine him cobbling together enough limits on deductions for high earners to offset the cost of the rest of the plan. That leaves effects on economic growth as the only way this could reduce, rather than increase, the deficit, and in practice income tax rates don't appear to matter much for long-term growth, and certainly not enough for big tax cuts to pay for themselves.

The distributional impact of the plan is also hard to judge. The cut to the top rate means that the rich will almost certainly pay considerably less, and the increase in the standard deduction and increase in the EITC suggest that low-income taxpayers will pay less as well. But it's unclear if people in the middle will gain from the rate cuts on average, especially because as of this writing, Bush hasn't specified the thresholds for his different rates. If someone currently in the 15 percent bracket were thrown by Bush into a 25 percent bracket, she could very well wind up worse off.

What Jeb is trying to say with this plan

You can understand Jeb's proposal as an attempt to negotiate a disagreement within the Republican Party about how best to cut taxes. On one side are the supply-siders, represented by conservative commentators like the Heritage Foundation's Stephen Moore, former GOP presidential candidate Steve Forbes, and CNBC's Larry Kudlow, who argue that cuts in marginal tax rates are the most important policy change for stimulating economic growth. (Bush, tellingly, consulted Moore, Forbes and Kudlow on the plan.) In Congress, this group is most vocally represented by House Ways and Means Chair Paul Ryan, whose budgets have called for a top rate of 25 percent. They also tend to like flat tax proposals, like those floated by Rand Paul, Ted Cruz, and Ben Carson (though Paul's is a bit of a weird variant on the flat tax).

On the other side are "reform conservatives" like economist Robert Stein and Sens. Marco Rubio (R-FL) and Mike Lee (R-UT), who think that the focus on rate cutting made sense when the top rate was 70 percent, as it was when Reagan took office, but that we're at a point of diminishing marginal returns from further cuts. Rubio and Lee's tax plan has a top rate of 35 percent — lower, but not as low as Ryan or the flat-taxers would like. Instead, Rubio and Lee enact or expand a number of tax breaks meant to help middle- and low-income families. They would create a new, mostly refundable child tax credit on top of the existing one They also turn the personal exemption into a credit and increase it substantially, which is a cut targeted to help poor and middle-class families, who get less out of deductions and exemptions since their tax rates are low.

The "reformocons" think an agenda focused on middle-class tax cuts is more politically viable than one focused on cutting the top rate, but they also genuinely think it's important for the future of the economy. They believe that social programs for the elderly like Social Security and Medicare reduce parents' need for support from their children, and thus reduce the incentive to have children, and thus threaten to reduce the number of future workers around to keep the programs running. The new child tax credit is supposed to correct for this alleged disincentive.

But the supply-siders think this is heresy, that the child tax credit is just another redistribution program that takes from the rich to help the poor, and that focusing on growth must mean focusing on cutting the top rate.

Bush would increase the dread "47 percent"

So Bush splits the difference. He has a large, real cut in the top rate to appease the supply-siders. But he also offers cuts for low-income and middle-class workers in the form of a bigger standard deduction and an expanded earned income tax credit. He doesn't include reformocons' signature child tax credit proposal, but these cuts feel reformocon-influenced.

These provisions are particularly notable because they — by Bush's own admission — would grow the "47 percent" of Americans who don't pay federal income tax (the real number for 2015 is more like 40.4 percent). While many in the conservative base sympathized with Mitt Romney's argument that low income tax burdens on the poor and middle class remove too many people from responsibility for funding the government, reformocons generally cringed at Romney's comments, and argued that growing the number of people who don't have to pay income taxes can be a good thing.

By promising to spare 15 million more households from income taxes, Bush would increase the share of Americans not paying income tax in 2018 from 38.3 percent to 47.6 percent, according to the Tax Policy Center's figures.

Jeb's corporate tax cut plan is mostly de rigueur for Republicans, but the immediate expensing of investments provision is notable. It effectively turns the corporate tax into a kind of consumption tax, which economists tend to think is better for economic growth. But there are some major implementation challenges with such a plan, which Rep. Devin Nunes (R-CA) faced when formulating a similar proposal in Congress in 2013.