Barack Obama’s tax plan will either raise $262 billion over the next 10 years or increase the national debt by $2.7 trillion. John McCain would add either $615 billion or $3.6 trillion to the debt.

What’s going on? Don’t everyone turn your computer off at once, but we need to talk about budget baselines.

There is nothing more esoteric, but Obama and McCain have made them hugely important. Trillions of dollars important. In fact, the only way either candidate can establish even a nanobit of fiscal credibility is by dramatically reframing the deficit discussion.

Both want to convince us that the Bush tax cuts will go on forever, even though they are due to expire in 2010, and that the Alternative Minimum Tax mess has already been fixed, although a permanent solution is nowhere in sight. With these helpful assumptions, their trillions of dollars in tax cuts look modest. Both candidates can make it appear as if they are merely moving around a bit of loose change, rather than massively increasing their grandchildren’s debt.

This is nothing more than a fiscal parlor trick. McCain, at least, can argue that he has supported the Bush tax cuts—well, he supported them after he opposed them. Obama has voted time and again against extending them and calls them irresponsible. What is likely to be a strongly Democratic Congress will never vote to sustain them as is. Yet, both Obama and McCain would like us to believe these tax cuts are cast in stone—the fiscal Ten Commandments, if you will—even as they propose to change them.

Neither seems to have noticed that Washington routinely overhauls the tax law every decade or so. When it comes to taxes, change is the status quo.

There is an easy way to cut through this palaver. Forget the baseline. Just think about three numbers: How much would either candidate collect in taxes as a share of the Gross Domestic Product? How much is government likely to spend? And, how much would they have to cut that spending to keep the national debt from ballooning.

TPC estimates that in 2013, Obama would collect revenues of 18.2 percent of GDP. McCain would bring in about 17.8 percent. Spending that year would be about 19.5 percent, according to the Congressional Budget Office, assuming the Iraq war will be winding down.

Thus, Obama would have to cut spending by 1.3% of GDP or $230 billion, to balance the budget in 2013. McCain must find 1.7% of GDP, roughly $300 billion. For context, Bush and the Congress have been battling for years over budget cuts one-tenth that size.

I await word on the candidates’ additional spending cuts. Obama has embraced costly new initiatives for infrastructure, education, health care, and energy, but said little about exactly where he’d cut spending. McCain vows to cut pork, which might get him 5% of what he needs. On the other hand, he is not likely to end the war any time soon.