Irwin "Tubby" Harrison, a Democratic pollster, has the wrong nickname. It should be "tasteless." Harrison is quoted in The Wall Street Journal, exulting over Ronald Reagan's performance in his debate with Walter Mondale: "It takes away that macho image. He's an old man." Shame on you, Tubby.

If Reagan is slowing down, if at 73 he's slipping off to senility, is occasionally befuddled, unable to come up with the precise word and verbally treads water by repeating himself, it's indeed a cause for concern and should be discussed. He is, after all, the president, and the job calls for mental acuity. But doubts concerning Reagan's age should be no cause for glee in the Democratic camp. In the first place, we all have our off nights. Maybe Reagan did not sleep well. Maybe he ate something that disagreed with him. Maybe, for some unexplainable reason, he simply was not himself.

Second, if Reagan is handicapped by his age then that is just plain sad. The man has accomplished an awful lot in his life. He entered politics late, a second or maybe third career for him, and it would be tragic if time -- sheer age -- robbed him of the chance to complete his work. But life, as John Kennedy once observed, is not fair, and there remains the possibility that Reagan is alarmingly past his prime. If that's the case, there's nothing anyone can do about it. No amount of wishing can change matters -- and no amount of naps or lackadaisical schedules can turn back the clock. There will be times when world events, and not Mike Deaver, will control Reagan's schedule, and the nation can ill afford to have a president who's literally asleep at the helm.

There have been suggestions that the president undergo tests for senility -- and make the results public. One of those tests entails doing mental feats such as counting backward from 100 by sevens. I tried that, got to 93 and gave up, concluding that's why the Japanese make calculators. I could not blame Reagan if he took that suggestion, placed it on his woodpile and chopped it to pieces. Anyway, as long as he's sitting on a lead, the president is not about to take that kind of test.

But the real test -- the test of the presidency itself -- is performance. If Reagan and the men around him really want to put the age issue to rest, they should spring Reagan from the Deaver-Baker-Darman bubble and show us what he can do. Let's see him really campaign. Let's see him make more than two stops a day. Most of all, let's see how the president performs if, like Mondale -- like almost any candidate in memory -- he takes questions from the traveling press. Then and only then will the voters know whether Reagan is up to the job.

So far Reagan has conducted no such campaign. His reelectioneffort has been a succession of staged events -- of balloon barrages and flags waving and hearty smiles for the television cameras. That kind of campaign tells us all the things we either know already or don't need to know at all -- that Reagan is personable, that he's a master at pageantry, that he's a wonderful grand master of any parade. What we don't know is if he's worth a damn after four in the afternoon.

The issue of age is a serious one, not beyond politics, but beyond cheap shots. Only Ronald Reagan himself can put it to rest. Let Reagan be Reagan. And let us see exactly what that is.