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.