In response to my blog the other day about economists endorsing John McCain’s proposal to create an alternative individual income tax, Winghunter asked a perfectly reasonable question: What would such a scheme do for the economy?

Winghunter was asking about a version proposed earlier this year by Fred Thompson, a plan which mimics one first put out by the House Republican Study Committee. But the idea is essentially the same: Individuals would figure their liability under both the regular income tax and a simplified lower-rate alternative and pay whichever is less.

TPC has concluded that such a Thompson-like tax system would reduce federal tax revenues by an eye-popping $7 trillion over 10 years. But, getting back to Winghunter’s question: What would that do for the economy?

The short answer is nothing good. A conventional lower-rate structure would boost growth, but only if it is financed, either by spending reductions or tax increases. It happens that CBO has just released a report that looks at the economic effects of a much more modest plan—permanently indexing the Alternative Minimum Tax and extending the 2001 and 2003 tax cuts. It estimated that unless these tax cuts are paid for, deficits would reach 5 percent of GDP by mid-century and 18 percent by 2082. Eighteen percent of GDP happens to be about what we collect in total tax revenues each year. Hello Argentina.

In this study, CBO director Peter Orszag says the economic consequences of such a flow of red ink are literally unimaginable. As he put it, “projected deficits would grow to levels well beyond the range for which economic models have been developed.” Diane Rogers over at economistmom.com has a nice take on this.

Of course, some on the Right may try to dismiss Orszag’s analysis since he used to work in the Clinton Administration and at The Brookings Institution—a think tank Winghunter dismisses as “liberal.”

Trouble is, Orszag’s analysis is essentially identical to what CBO was saying 5 years ago, when its director was Doug Holtz-Eakin, a highly-respected conservative economist who is now John McCain’s chief economic adviser. This is what he said about the impact of tax cuts that are not financed:

“Sustained and rising budget deficits would affect the economy by absorbing funds from the nation’s pool of saving and reducing investment in both the domestic capital stock and foreign assets… As a result, the growth of workers’ productivity would gradually slow, real wages would begin to stagnate, and economic growth would tend to taper off. If that situation continued long enough, rising deficits could actually lead to a sustained contraction of the economy.”

So, no problem. All we need to do is find a way to cut $700 billion-a-year from the $3 trillion federal budget. Until we do, it is pretty clear that tax cuts of this magnitude are nothing but bad for growth.