Sens. Marco Rubio (R-Fla.) and Mike Lee (R-Utah) are back with their tax reform plan, which they claim is a lot simpler.

There's a lot of moving parts to it. But one key part is consolidating the seven brackets in the income tax down to two: a 15 percent rate and a 35 percent rate, with $87,850 ($175,500 if you're married) as the cut-off between the two. "It's a flatter tax system," Rubio told a meeting of Club for Growth donors, Politico reported. "In an ideal world, it would be a simple one rate for everyone. Hopefully we'll move in that direction as a nation. We think this is achievable in the short term."

"If I got to start our country over from scratch, I would either have a flat tax or a consumption tax," he continued.

A flatter tax system is one of those perennial Republican quests. But it's a solution that doesn't match up terribly well with the problems Rubio, Lee, and their co-travelers say they're concerned with.

The basic idea behind the progressivity of the current system is it protects the least well-off. If you're a single individual making $9,000 a year, every cent you earn is going to food, shelter, clothing, etc. If you make $38,000, that first $9,000 is still really crucial, but the next $29,000 slightly less so, and so on. Which is why each one of those batches of money is taxed at a progressively higher rate: the money you need the most gets taxed the least.

And while Rubio and Lee's plan benefits the richest the most, it basically amounts to a tax cut for everyone. So if all you're concerned with is the tax burden of middle-to-low earners, it's not a bad deal. The problems are elsewhere.

One, Rubio pitches the plan as a blow in favor of simplicity, which makes no sense. The current setup of seven brackets may seem complex. But once you know what your taxable income is, it takes a page or two of forms and a few minutes of simple arithmetic to figure out your tax liability.

The complexity, rather, comes in figuring out your taxable income. There are a host of deductions and exemptions in the tax code, all of which remove some of your income from the "taxable" bucket. And afterwards, when you've determined how much you owe, there are various credits that reduce that number, too.

A flat tax — just one tax bracket with one rate for everybody — would still be an income tax, and thus defining "taxable income" would still be necessary. Politicians could add just as many deductions and exemptions and credits as we have now.

A consumption tax — i.e. a national sales tax of some sort — presents a similar problem, in that you can still introduce all sorts of carve-outs. Its big advantage over a flat tax is there's no tax paperwork for individuals; they just pay the sales tax whenever they buy a good or service, while the business handles the paperwork.

How simple the U.S. tax code is depends on what loopholes you introduce. Every so often, Congress goes through the code and clears out a bunch of loopholes that have built up. But then the natural grind of politics and lobbying and interest groups inevitably reasserts itself, new loopholes are created, and eventually Congress has to go back and clear it out again. It's like scraping the barnacles off a ship's hull. It's never actually done, and whether you have one tax bracket or 50 doesn't change the process.

The other problem with Rubio and Lee's plan is revenue. Even with the loopholes they close, they lose $2.4 trillion over the next decade. And we know how the GOP would balance the books: by slashing the social safety net.

This is really the bedrock problem with Rubio's "ideal" of a flat tax or a consumption tax: the only way to raise enough revenue is to jack it up high enough that you're doing real damage to the poorest people in society.

Now, Europe uses consumption taxes quite often, mixed in with progressive income taxes, and the overall result is a tax burden that's considerably more regressive than America's. But Europeans balance this out by raising lots more revenue than the U.S., and then spending it on robust public services and massive downward transfers of income. So the poor in their societies actually wind up doing considerably better than in America.

In essence, Lee and Rubio want a European-style tax system, but they don't want a European-style safety net. To the extent they're willing to tolerate social spending at all, they want it small and tightly aimed at the most needy. They're trying to match the wrong type of safety net with the wrong type of tax system.

If you're going to have a small safety net targeted at the bottom, you need a small tax base targeted at the top. Which is where America's income tax has essentially wound up anyway: we've blown so many holes in it with carve-outs, credits and whatnot that roughly the bottom half of earners pay nothing. (They still pay state and payroll taxes.)

The perversity is, they still have to go through the annual rigmarole of their income tax forms in order to eliminate their liability.

If Rubio and Lee were feeling really reformist, they'd just acknowledge this situation for what it is, and propose a reform in which the income tax doesn't even kick in until median household income or higher. They'd need extra brackets with really high rates on the truly rich, to raise the revenue for the lean and mean safety net they want. But the bottom half of Americans would still have no income tax liability, and they wouldn't even have to bother with any tax forms come April 14.

That's simplicity for you.