IT'S BEEN a whole day since Democratic presidential candidate Bruce Babbitt proposed a national sales tax and, so far as anyone has been able to tell, no bolt of lightning has come to fetch him down. We mention that not because we like the idea of a national sales tax -- we don't -- nor to mock the political risks still involved in proposing a tax increase -- they are real. You could argue further, as perhaps his opponents quietly will, that Mr. Babbitt, lagging badly in the polls, has very little to lose. He nevertheless deserves enormous credit for confronting an issue that his rivals, though generally of like mind, have tended to fudge.

The voters may dislike the idea of a tax increase, but they aren't dumb. Just about everyone recognizes that the level of government services is now about as low as it is likely to go -- perhaps, though everyone has a program or two that he himself would cut, about as low as it should go. Meanwhile, we aren't paying the bill. In this presidency or, if Mr. Reagan won't clean up after his own parade, the next, there needs to be a sizable tax increase.

A national sales tax is the wrong way to achieve this because it would fall indiscriminately on those who can and cannot afford to pay. That's why the still somewhat progressive income tax is better (even though, in theory at least, an income tax militates against savings and investment, which we need and which a consumption tax might promote). Mr. Babbitt suggests that this objection could be met both by exempting food and other necessities from the sales tax and by compensating for the sales tax through an increase in the income tax threshold. Maybe someday it will come to that, for want of an alternative that can be passed, but that case hasn't been established.

For now the kind of tax Mr. Babbitt proposed is less important than the fact that he forthrightly proposed one. The myth is enshrined that Walter Mondale lost the last election in significant part because he endorsed a tax increase and therefore that no Democrat can again afford to do so. We think that 1) Mr. Mondale lost the last election for a lot of reasons, of which his stand on taxes was only one, and 2) the last election is not the next. The need is both to be honest about and to legitimize the step that almost every knowledgeable person agrees must be taken. That's what both campaigns and leadership are for. Some of the leading Democrats in Congress -- House Speaker Jim Wright is perhaps the most notable example -- have also stepped up to the tax issue. That bolt of lightning hasn't struck them, either. What if, once again, the people turn out to be smarter than the politicians think they are?