Even with its favorable assumptions and wide-ranging deficit-reduction measures, their proposal would still leave the national debt in 2020 at the same unprecedented level it is today—equal to more than 60% of GDP. With their plan, it would take until 2035 to bring the debt-to-GDP ratio back to the 40% level we had just two years ago.

That's not soon enough. The mere prospect of persistent high deficits jeopardizes the current recovery by creating the expectation that tax and interest rates will eventually rise substantially. It also increases the risk of a sudden financial crisis if foreign investors decide to stop buying U.S. government debt. And in the longer run, persistent deficits will indeed lead to higher taxes and higher interest rates that will reduce business investment and economic growth.

The commission chairmen are correct to reject higher tax rates as the way to cut future deficits. Instead, they spotlight reducing tax expenditures—that is, the spending, through the tax code, that now subsidizes mortgage payments, local government spending, health insurance and much more at a total annual cost of $1 trillion. But rather than reform tax expenditures in order to finance major deficit reductions, the chairmen propose to use most of the revenue to lower tax rates, leaving only $80 billion a year for deficit reduction. Combining tax-expenditure reform and lower tax rates is good, but the proposed mix does too little deficit reduction.

Reducing the long-run growth of the national debt requires reforming Social Security and Medicare. The key is to shift from today's tax-financed system to a mixture of tax-finance and universal saving accounts, which would supplement the tax-based payments. The chairmen omit any proposal to achieve universal accounts.

For Social Security, they discuss benefit cuts but not the additional universal saving accounts needed to protect the income of future retirees from falling substantially relative to the incomes of the working population.

For Medicare, the chairmen's proposal is sound: limiting the growth rate of government health-care spending to 1% more than the growth of GDP, while letting individuals supplement the government program with additional insurance or out-of-pocket funds. That approach recognizes that the real health-cost problem is not the rising level of future health spending but the tax-rate consequences of financing that increased spending with higher taxes.

As national incomes continue to rise, future generations will want to spend more on health care. Funding that through the tax system would unnecessarily hurt the economy and slow economic growth. Universal saving accounts could give retirees the funds needed to supplement the basic Medicare program. The alternative approach of limiting the total amount of health care that doctors and hospitals can provide, as many advocate, would deny future seniors the improvements in quality and duration of life that medical science will achieve.

The deficit commission chairmen round out their deficit-reduction proposal with dozens of specific cuts in "discretionary" programs—those that, unlike Social Security and Medicare, require annual appropriations. For the year 2015, the proposal suggests more than $100 billion of illustrative cuts in defense programs and an equal amount in nondefense programs. But why should defense and nondefense spending be reduced equally? While defense and nondefense spending have approximately equal shares of total "discretionary" spending, defense is less than one-fifth of total budget outlays. The proposed cut in defense spending looks more like a political deal than a careful calculation of defense needs at a time of increased risks to U.S. national security.

Surprisingly, the chairmen overlooked the easiest route to reducing the deficits over the next decade: scaling back the costly budget that President Obama presented earlier this year. Much of the projected doubling of the national debt between 2010 and 2020 reflects the spending and tax proposals in that budget.

The Congressional Budget Office estimates that those proposals would, if enacted, raise the 10-year budget deficit by $3.8 trillion, even after taking into account the president's proposed $1.3 trillion of new taxes on businesses and higher-income individuals. The $5.1 trillion gross cost of the Obama proposals reflects the cost of making the Bush tax cuts permanent for individuals with incomes below $250,000, of providing additional tax cuts for low- and moderate-income individuals, and of increasing spending on domestic programs.

As President Obama considers the bipartisan commission's proposals and plans his next budget, he should begin by removing some of the $3.8 trillion of increased deficits that he proposed earlier this year. Financial markets and policy makers around the world want to see if the administration is as serious about deficit reduction as the American public.

Mr. Feldstein, chairman of the Council of Economic Advisers under President Reagan, is a professor at Harvard and a member of The Wall Street Journal's board of contributors.