Insurgent political movements are usually built around a big idea, like abolition or workers' rights. The Tea Party certainly has a big idea: Shrink the government.

Wanting to shrink the government is a perfectly reasonable impulse given the state of Washington's finances. The federal debt has more than doubled as a share of GDP since 2007, and future spending projects are off the charts. The latest academic evidence suggests an increase in government size is associated with slower annual GDP growth.

It's easy to see why this shrink-the-government idea is powerful, and how it fueled the Tea Party's rapid ascent into a rocket-powered force on the right.

However, a big idea alone is not sufficiently enough, in and of itself, to guarantee success. And therein lies the Tea Party's big problem.

The Tea Party's blueprint for turning their raison d'être into reality is flawed. Called the "Penny Plan," it's a favorite of the Tea Party Patriots, media supporters such as Sean Hannity of Fox News, and fellow travelers in Congress, including possible 2016 presidential candidate Rand Paul and — perhaps most importantly — Mike Enzi, the new Republican chairman of the Senate Budget Committee.

First devised by Georgia businessman Bruce Cook, the Penny Plan would cut government spending by 1 percent a year until the federal budget is balanced. After that, federal spending would be capped at 18 percent of GDP, to match the long-term revenue trend. Here's how Enzi touts the plan on his website:

Though only a 1 percent cut, the savings add up quickly to balance the budget. And if it's done right, where we're eliminating duplication and sensibly prioritizing, discomfort will be manageable. … Living with 1 percent less is a small price to pay in order to help bring this country back from the brink of catastrophic fiscal failure. [Enzi]

It sounds so simple! Well, it really isn't.

For starters, the "penny" part of the plan is a gimmick, more about marketing than math. The Enzi version would cut 1 percent a year from total government spending, other than debt interest payments, for three years. Maybe that doesn't sound like much. But once you factor in inflation, that works out to a 10 percent cut in real terms after three years.

Now maybe that still doesn't sound like much. But getting such a reduction is tough enough that there are no details in the Penny Plan about what exactly would be cut. To balance the budget in 2018, according to CBO, it would require $540 billion in reduced spending. It can't all come from reducing non-defense discretionary spending such as foreign aid or scientific research. That part of the budget, just 17 percent, or around $600 billion, is already at its lowest levels since the 1960s as a share of GDP.

That leads to a bigger problem with the Penny Plan: Is it realistic to cap long-term government spending at 18 percent of GDP — well less than the post-WWII average of 21 percent — when an aging population means increased spending on entitlements such as Medicare and Social Security? Remember, most of the spending increase from health-related entitlements and Social Security — 75 percent over the next quarter century — comes from simple demographics, more people getting benefits over a longer period of time. That works out to about 3 percentage points of GDP in additional spending baked into the budgetary cake. Overall, CBO projects total spending at 26 percent of GDP by 2039.

Just keeping long-term spending at its historic average will be a huge challenge, much less sharply reducing it. If you also want to spend a bit more on important public investments such as infrastructure and basic research while keeping military spending constant — well, good luck. Even the GOP Senate's new balanced budget amendment — which doesn't calculate debt interest payments as spending — would have a tough time hitting its 18 percent target.

That the Penny Plan offers zero specifics on how to make the numbers work undercuts its seriousness. It would obviously require sweeping entitlement reform — and more. But Enzi, for one, argues that "we should focus on identifying and eliminating all of the wasteful spending that occurs in Washington before we look to other important programs and services." That's an evasion, though hardly a surprising one from a party that depends on older voters.

In fact, some on the right are trying to fudge that political reality by distinguishing between "earned" entitlements — Social Security and Medicare — that go to GOP-leaning voters and "unearned" entitlements — such as Medicaid and ObamaCare subsidies — that go to Democratic-leaning voters.

So yes, the Tea Party has a big idea. But it has no idea how to make it happen.