Walter Mondale fell on his sword when he took on the President personally in the final days preceding the primary. When Mondale talked of Reagan's ''amnesia,'' he didn't take into account that the President is very popular in New Hampshire. And voters don't take kindly to an attack on their President, even if they are Democrats and may have some reservations about the things Mr. Reagan is doing.
Meanwhile, Gary Hart was swinging at the administration in a more general way , saying things that could be interpreted as political. Undoubtedly the ''momentum'' - a word political observers are fond of using - was with Hart after the Iowa caucus, but Mondale certainly didn't slow it down by affronting those Democrats who have a liking for the genial President.
Mondale should remember that one of the reasons John F. Kennedy won that squeaker against Richard Nixon was that he never made any disparaging remarks about the man who had headed the administration he was running against: President Eisenhower.
Kennedy's instincts told him to stay away from Eisenhower - that the popular Ike retained a formidable constituency that included Democrats and independents. Instead, Kennedy concentrated on the future and the ''new ideas'' which have become the Hart thesis.
What Mondale was doing with his ''amnesia'' thrust was more direct than he may have realized. In so raising the age issue about Reagan he broke a cardinal political rule: To charge an opponent with being too old is considered to be an approach that will almost automatically evoke widespread sympathy for the target of the accusation. Further, it will cause a lot of voters to conclude that the candidate raising the issue is taking the low road.
If the age issue is to work against an older candidate, it must emerge naturally in the campaign. It must come out in that candidate's performance, perhaps in speeches or, in the way he conducts the public office he holds.
Mr. Reagan is, however, looking very keen and robust these days, as anyone who watched his recent press conference could attest. He may ramble in his answers. But that's the Reagan style as he cautiously avoids the precision that might get him into trouble.
Reporters meeting with Reagan recently found him full of vitality. The day before there had been false reports about his health that had, for a few hours, shocked the stock market until denied by the White House.
This is a President who still insists he never takes naps. When Reagan was a candidate in 1980, a veteran reporter asked him about this and he said: ''When I think about a nap, I just go out and chop some wood.'' A few hours later, on the Reagan plane, he sent a note back to the reporter: ''Have you had your nap yet?'' he asked.
The so-called ''conventional wisdom'' in Washington is that only a sudden appearance of deteriorating health will deprive Mr. Reagan of reelection. On the other hand, this hand-wrestling President, with body language of a 50-year-old, gains political points - or call it admiration - for being a vital, unflagging figure at age 73.
Parenthetically, an observation seems in order here: People in this country are, indeed, increasingly being judged by their vitality, their ability to get their job done, and not by their age. Certainly, Reagan didn't start this trend. But his example is helping to carry it along.