$157 a month. If that figure doesn't have some resonance for you, the National Republican Congressional Committee has been wasting a lot of money. In the past week or so, the NRCC has been running 30-second spots on national television charging that Walter Mondale's tax plan "would cost the average household" $157 a month. A perfectly fair, straightforward attack -- if the claim were true. But, it's not.

It's untrue even if you accept the Republicans' premises and agree with their cost-accounting numbers rather than with Mr. Mondale's. The Republicans argue, for example, that in costing out Mr. Mondale's tax plan, you should not simply count the amount he says he will increase taxes, but should take into account the extra spending and taxes necessary to fulfill campaign promises Mr. Mondale has made along the campaign trail. There is nothing wrong with that as a general proposition, and everyone understands that putting a price tag on a politician's promises is not an easy matter even for a neutral observer; there is room for argument over the dollar figure.

Where there is not room for argument, however, is over the fact that Mr. Mondale's proposed tax increase is tilted toward high-income taxpayers. It may be accurate to say that the average tax increase per household under his plan would be $57 or $157 a month. But that's quite a different thing from saying that the average household would have its taxes increased by that amount. That's like saying that the average American family gets so-and-so dollars a month in welfare payments. The average family gets nothing at all from welfare, although if you take total welfare payments and divide them by the number of families you could say that the average payment per household is so-and-so.

So the Republican ad gives a misleading and unfair impression, even by the standards of campaign hyperbole. Currently the ad is not running, and the Mondale campaign says that two of the three networks have agreed not to broadcast it again without substantiation. The NRCC, if it doesn't want to apologize, should take the wise course of not raising the issue and leaving the $157-a-month spot alone for the last days of the campaign.