YOU WOULD think it was the last week of the campaign. Breathlessly, the polltakers tell us that the longtime front-runner has been overtaken by an attractive challenger. We are in the stretch run at the Preakness, it seems--will Deputed Testamony be able to pull out ahead of Desert Wine? Except that the horses are nowhere near the stretch. In fact, we're more than 17 months away from the 1984 general election and more than nine months away from the February and March precinct caucuses and primaries that will probably determine who wins the Democratic nomination. That's worth remembering, hard as it seems for some.

It's true that John Glenn took the lead over Walter Mondale in a recent Los Angeles Times poll by a 28 percent to 26 percent margin, and that in the Gallup poll Mr. Mondale's lead of March (32-13) was sharply reduced by the beginning of May (29- 23). But those figures don't mean that the race is almost over--or even that it has really begun.

For what do voters know about these candidates today? Not much; far less than they will know when they will be called upon, early next year in the caucuses and primaries, to make a choice. They know that Mr. Mondale was vice president, that Mr. Glenn was an astronaut and that both men have served in the Senate. But they know little about their character, their records in office, or their priorities or positions on issues. They know even less-- in some cases nothing--about the other four men who are seeking the Democratic nomination.

The poll figures, given the margins of error, suggest that Mr. Mondale's support stayed about even in March and April and that Mr. Glenn's grew. The only event that might account for the Glenn surge was his announcement of candidacy April 21; his staffers hypothesize that voters who remember his achievements and admire his character may have swung into his column when they learned he was a candidate. That's plausible. But if a vote can be swung by a two-minute segment on the evening newscast and a few memories, it probably isn't very solid yet. We suspect that most of the support for any of the Democratic candidates today is not strongly committed, and that almost all the voters are ready to switch for any good reason.

It may be they won't find one, and the results of the first caucuses and primaries will look a lot like this month's polls. But, as any pollster will tell you, polls aren't predictions; they're just pictures of opinion at one time. And in this case the picture is drawn in very soft and wispy tones.