Donald Trump has come up with the most mathematically impossible plan, well, possible.

That's not hyperbole. Trump wants to cut taxes by $7 trillion the next eight years, doesn't want to cut Social Security, Medicare, or Defense one penny, and still wants to pay off the $19 trillion national debt by the end of his hypothetical second term. About the only thing he hasn't promised is that he'll try to top Oprah by giving everyone a car and a pony.

There's just no possible reality where these numbers could even come close to adding up. Consider this: the Congressional Budget Office expects that the federal government will take in $32 trillion in taxes between 2017 and 2024, but the nonpartisan Tax Policy Center estimates that Trump's tax plan would reduce that by $7.2 trillion over that time. So that leaves $25 trillion to pay for all the government and the $19 trillion national debt. Or, with a little subtraction, just $6 trillion for the government during a full two-term Trump presidency. But that's not even enough for Medicare. Indeed, the CBO tells us that Social Security, Medicare, and Defense—which, remember, Trump has promised not to cut—will cost $21 trillion by themselves those 8 years.

In other words, Trump would only have $6 trillion to pay for $21 trillion—and that's assuming he eliminated the rest of the federal government other than Social Security, Medicare, and Defense. That means no more spending on roads, bridges, schools, scientific research, food stamps, unemployment insurance, Medicaid, or Obamacare subsidies. That's not a plan. It's a joke.

But it's the kind of joke that Republicans have been telling themselves for awhile now. Especially House Speaker Paul Ryan. He wants to slash the top tax rate to 25 percent, doesn't want to cut Social Security or Medicare for today's seniors, actually wants to increase defense spending, and still wants to get rid of the deficit (though not the debt) in the next 10 years. If that sounds familiar, that's because it is: Trump has just taken this and turned it up to 11.

Let's back up a minute, though. How does Ryan think he could pull off his own slightly less unrealistic plan? Easy: with magic. Ryan's first trick is to assume that his tax cuts wouldn't cost a thing instead of the $5.7 trillion that the Tax Policy Center says they would, since he would supposedly close enough tax loopholes to pay for them all. Of course, he doesn't actually say which ones he'd get rid of. All he says is that he wouldn't touch the charitable tax deduction. The only problem, as the Tax Policy Center's Howard Gleckman puts it, is that it's "hard to imagine" that this is even possible. You'd have to go sacred cow hunting, with the mortgage interest deduction among the likely casualties. Not only that, but this kind of cut-tax-loopholes-to-pay-for-tax-cuts plan couldn't help but lower taxes on the rich at the same time that it'd raise them on everybody else. No wonder Ryan doesn't want to spell this out.

Then there is Ryan's spending trick. That isn't turning Medicare into a less generous voucher system for future retirees or even turning Medicaid into a much less generous block grant. No, it's assuming that he could get the government to all-but-stop spending money on the government's most basic functions. Without saying where or how he would cut, Ryan simply asserts that he could bring the government's non-Social Security and non-Medicare spending down from 12 percent of GDP today to 3.75 percent of GDP in 2050. Does it sound plausible that the government could spend three times less on—here we go again—roads, bridges, schools, scientific research, food stamps, unemployment insurance, and defense than it does right now as a share of the economy? It shouldn't. Not unless you think voters wouldn't care if the government stopped doing the things they want it to do.

In short: Ryan's plan uses a magic asterisk to try to cover up how politically impossible it is, while Trump's plan is a magic asterisk that doesn't come close to covering up how economically impossible it is.

But here's the irony. Ryan's unspecified savings and implausible assumptions have somehow turned him into the GOP's intellectual authority at the same time that Trump's have turned him into a laughingstock. Why is that? Well, Trump's mistake has been saying that he could achieve the impossible in 8 years instead of 35. The smaller your budget window, the harder it is to hide that your math doesn't work because it has to be an order of magnitude crazier.

Not that Republican voters seem to care. After all, why would you believe that one mathematically challenged plan is an embarrassment when you've been told that another one is a tour de force?

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Republican presidential candidate Donald Trump met with The Post's Bob Woodward and Robert Costa to discuss his campaign. (The Washington Post)