Last week, Standard & Poor's lowered Japan's bond rating to AA-, the fourth-highest level. By that standard, the U.S. got away with a slap on the wrist from Moody's Investors Service, which warned merely that "the probability of assigning a negative outlook in the coming two years is rising."

If you look at the U.S. budget trajectory with an eye on the lessons from Japan's recent history, there's a strong case that the U.S. rating should be cut immediately.

It's true that the U.S., with total government debt equal to 98.5 percent of gross domestic product, according to Organization for Economic Cooperation and Development data, has many years of unrestrained deficits ahead before it reaches the crisis point of Japan, which has debt of 204 percent of GDP.

A more plausible target, however, is 135.4 percent of GDP. That was Japan's debt in 2000, just before S&P first downgraded it from AAA in February 2001.

If the U.S. makes no fiscal progress, and continues to run annual deficits at the 2011 level of $1.48 trillion dollars, it will take just six years to reach a debt level of 135.3 percent of GDP. The Japan precedent suggests the U.S. would lose its sacrosanct AAA rating at that point, if not sooner.

To be fair, the Congressional Budget Office, in its forecasting, predicts that the U.S. will do better than that, in part because revenue should increase as the economy recovers.

CBO's wholly unrealistic baseline forecast suggests the day of reckoning is far off. Don't believe it.

Doctors' Pay

The budget agency's somewhat more grounded "alternative fiscal scenario" reflects "fixes" likely to be passed by lawmakers, such as higher payment rates to doctors under Medicare. If we use this alternate forecast, and factor in CBO assumptions that high debt levels would crowd out capital investment, then the U.S. hits the 135 percent debt mark in 2020 or 2021.

Be happy that Japan, not Greece, is the logical point of comparison here. Greece saw its credit rating downgraded repeatedly in 2009. Its debt was 105.6 percent of GDP at the end of 2008 and 120.2 percent of the nation's economic output at the end of 2009.

The U.S. could depart from the collision course with a downgrade if it took serious steps to reduce its deficit. But President Barack Obama's State of the Union address offered pitifully small spending cuts while floating Obama's fiscal commission out to sea on an iceberg.

I may be old-fashioned, but all of this should mean that rating of U.S. long-term debt should be downgraded--today.

Uneasy A

A report from S&P last October estimated that "absent policy and other changes" the U.S. could be rated A--two rungs below Japan's current status--by 2020. So start the process now, for goodness sake.

Think of it this way: Somebody buying and holding a 10-year or 30-year U.S. bond today faces a pretty good likelihood of suffering a downgrade. Shouldn't a bond's rating have a strong chance of staying the same over the life of the bond?

I should add two important caveats:

First, gross government debt numbers might not be the best measure of country-specific risk. A U.S. with debt of 135 percent of GDP might reasonably still be a safer place to stick money than an equally indebted country. Still, governments everywhere play the same kind of trust-fund games the U.S. plays, so the OECD data provide the best readily available apples-to-apples comparison.

Types of Debt

Also, focusing on gross debt may make the situation look better than it really is for the U.S., since Japan's is so high relative to debt held by the public. Japan's publicly held debt was 66.3 percent of GDP when it lost its AAA rating, compared with the current U.S. level of 72.7 percent.

Second, one might argue that it is unthinkable that the U.S. would ever default, so it should always have a solid AAA rating.

Think again. As debt levels rise, a fiscal situation can become simply impossible, making default inevitable.

And it can happen here.

Since its founding, the U.S. has defaulted on its debt twice. The first time, in 1790, it deferred payments on money owed stemming from the Revolutionary War. While the dollar values were eventually repaid--10 years later--the delay was technically a default.

No Gold

The second time, in 1933, the U.S. blatantly broke its obligation with creditors by refusing to repay loans in physical gold. Existing contracts stated that lenders could request payment in either dollars or gold, but President Franklin Delano Roosevelt and Congress, with plans to significantly inflate the dollar, passed a law forbidding repayment in gold. Again, the dollar values were repaid, but unilaterally changing the terms of a contract is the equivalent of a default.

The failure of the ratings companies to identify the riskiness of real estate loans clearly contributed to the financial crisis. If, given the evidence, they fail to act responsibly and change the U.S. credit rating soon, they will be setting investors up for yet another fall.