GOV. GEORGE W. Bush of Texas leads the Republican presidential field so comfortably that he could afford to skip last night's debate. On the Democratic side, by contrast, a tight contest has induced both candidates to submit to an exhaustive schedule of joint appearances. The first, which took place on Wednesday in New Hampshire, was civil and serious for the most part. Both candidates came off looking attractive, and like each other, and even liking each other. But the resemblance was unfortunate in at least one respect. Neither Al Gore nor Bill Bradley is being honest about the cost of his proposals.

At one point in Wednesday's contest, Mr. Gore accused Mr. Bradley of vastly understating the cost of his health reform. The Bradley plan claims to extend health coverage to 95 percent of Americans at a cost of between $50 billion and $65 billion a year. Mr. Gore retorts that it might cost $120 billion annually. He probably has half a point: The Bradley estimates, notably those for the cost of prescription drugs, are indeed low, and Mr. Bradley has to defend himself by talking vaguely about the administrative savings the Internet may deliver.

Mr. Gore, however, is not perfect either. He announced on Wednesday that his own health reforms would extend coverage to "almost 90 percent" of Americans at a cost of some $15 billion a year. Since 84 percent of Americans are currently insured, Mr. Gore is effectively promising to increase coverage about half as much as Mr. Bradley but for a fraction of the cost. This sounds incredibly clever -- and we emphasize incredibly.

Both candidates, moreover, like to say that their proposals can be paid for out of the non-Social Security budget surplus. It is true that this surplus has been projected at $1 trillion over the next decade, which would be enough to cover much of what each candidate wants -- or to pay for the $792 billion worth of tax cuts that Republicans have advocated. But the surplus projection is based on the assumption that Congress will respect the spending caps agreed upon two years ago. There is not the slightest chance of this. If it were not for ruthless accounting tricks, Congress would have formally broken the caps already.

If the candidates keep dissembling about costs, they risk squandering voter trust. More than that, they may damage the idea of constructive government that they profess to believe in. In most areas of social policy, politicians can make their ideas look cheap by a variety of tricks. They can, for example, use mandates to pursue their goals: Private firms can be ordered to raise wages or provide health insurance, at no cost to the federal budget. But these mandates are often cumbersome to administer; and they amount to indirect taxes that are more opaque and onerous than the direct sort. If government is to function well, politicians need to ask what works, not what disguises the cost of their ambitions.

Mr. Gore and Mr. Bradley want to do worthwhile things: extend health coverage, reduce poverty, look after the environment. They should have the courage to admit that this will be expensive.