A big and steadily increasing number appears on the U.S. Treasury Department's website. At mid-afternoon yesterday, it was $9,168,000,000,000 and change - a lot of change. It is, the department says, the national "Debt to the Penny and Who Holds It." It's not a number the Democratic presidential candidates spend much time talking about, but they should.

All the candidates have promised to abide by Pay As You Go, the part of the 1990 budget act that requires Congress to offset all tax cuts or increases in spending with spending cuts or revenue increases. But promising to abide by PAYGO, as it's called, isn't enough. The Democratic candidates, who have proposed programs that cost hundreds of billions of dollars, should tell voters not just how they will pay for their plans but also how they will reduce the national debt.

The Monitor's Sarah Liebowitz outlined the Democratic candidates' major proposals and how they claim to pay for them. The plans are lofty: Provide affordable health care, give tax breaks to the poor and middle class, fund research into environmentally sound sources of energy, improve public education, address the shortage of affordable housing and more.

Rolling back the Bush tax cuts for wealthy Americans is the most often cited way to get money to pay for those proposals, but the numbers don't add up.

Sen. Hillary Clinton, using Congressional Budget Office figures, says eliminating the Bush tax exemptions for households earning more than $250,000 per year would raise $52 billion annually. Gov. Bill Richardson says rolling them back for the wealthiest 2 percent of taxpayers would raise $40 billion. But their health care plans would cost much more than twice as much.

The balance could be made up by making the health care system more efficient, convincing Americans to lay off the donuts and investing more in preventive medicine. But it would be a mistake to count on those savings, says Robert Bixby, executive director of the Concord Coalition, the debt reduction lobby co-founded by former New Hampshire senator Warren Rudman. Making the system more efficient could mean that more health care gets delivered, which could reduce any savings, Bixby said.

On the spending side, Bixby wants to hear the Democratic candidates promise that if they can't find the savings, they won't enact the program. Republican candidates, whose campaign promises tend to center around even bigger tax cuts, should do likewise on the revenue side. He's right.

The economy could grow so fast that the debt becomes a smaller and smaller problem. But if not, and it keeps growing, paying interest on the debt will mean either huge tax increases for future generations or drastic program cuts and a limited ability to invest in the nation's future.

The debt can be paid off. Under President Bill Clinton, who had a lot of help from a booming economy, it was on track to be wiped out by 2006. Instead it grew by $3.4 trillion under President George Bush.

The Democratic candidates want to do a lot of wonderful, and costly, things. But their promises come with a string that's attached to the future of the nation's children and grandchildren. The candidates should be as committed to reducing the national debt as they are to making sure that every American has access to decent health care. There's still time for them to explain how they plan to do it.