While campaigning for the Democratic Party's nomination for the presidency, Walter Mondale spent much of his time denigrating his rivals for the nomination and his future opponent.
Now, with the actual campaign opening, he must do something more difficult than denigrating rivals. He must sell himself as a plausible alternate president.
There are today plenty of American voters who are inclined to vote against Mr. Reagan. The list begins with those concerned about the Reagan deficit, concerned over what they consider to be the mismanagement of American relations with the Soviet Union, and concerned about the size of the defense program. Add environmental conservationists, feminists, blacks, welfare recipients, the unemployed, and persons concerned about Mr. Reagan's possible appointments to the Supreme Court.
But there is a difference between a voter's feeling an inclination to vote against an incumbent president and actually taking the positive step of voting for the alternate candidate.
Those who are radically inclined will find it easy, but how many of the disinclined are radical about it? Many, perhaps a majority who have reason to vote against Mr. Reagan, would at present hesitate to go all the way and vote for Mr. Mondale.
It is Mr. Mondale's task to overcome that hesitation in order that all those who have positive reasons to want to vote against Mr. Reagan will feel free to do so.
How does Mr. Mondale do that? For one thing, he might steal a page from the Reagan campaign of 1980.
Back in that 1980 campaign there were many voters who had their own individual or group reasons for being dissatisfied with Jimmy Carter's record as President. But at the beginning many of them would have refrained from crossing over and voting for Mr. Reagan for fear that he might be too much a radical of the right, too hard in his conservatism and his militarism.
Well, before the campaign began Mr. Reagan was dissolving that hesitation by his homey, comfortable, soft-shoe nice-guy manner and behavior. He didn't sound angry. He didn't vow crusades against any large segment of the population. He disarmed doubt and fear.
The experts can argue about when the easy manner and nice-guy image began to work. I noticed it first during that Manchester, N.H., forum where Mr. Reagan wanted to turn a scheduled two-man debate between himself and George Bush into a larger affair including the four other Republican hopefuls. Mr. Bush tried to keep the others out. Mr. Reagan insisted that they be let in, saying ''after all , I am paying for this microphone.''
He came through as being fair and friendly, and in command of the stage. Mr. Bush lost face and never recovered from the episode.
Others think the turning point was in the debate with President Carter when he said, ''There you go again.''
Whenever and wherever the specific moment of change for individual voters, the essential fact is that by election day of 1980 those voters inclined to vote against Mr. Carter had decided that they felt free to do so. Mr. Reagan won by coming on during the campaign as Mr. Nice Guy.
Mr. Mondale has proved that he can identify and excoriate Mr. Reagan's shortcomings. But Mr. Mondale has not yet identified himself as a plausible alternate for the presidency.
Do we feel confident in our hearts that he is big enough and competent enough and wise enough and tough enough to do the job?
He might still lose even if he succeeds in doing all these things. Mr. Reagan has a long lead and enjoys undoubted popularity. There may not be enough of the disinclined to elect Mr. Mondale even if all of them decide to vote against Mr. Reagan. But Mr. Mondale must dissolve those doubts about his own competence to govern if he is to have a chance of winning.
While we wait to find out, I am going off on a holiday. I'll be back writing again in mid-October.