The dour, defensive, and evasive Fritz Mondale of the Democratic Party's primary campaign has been replaced by a political warrior who has visibly enjoyed the first week of the real campaign.
And why not? To almost everyone's surprise Mr. Mondale had successfully seized the campaign initiative and put Ronald Reagan on the defensive. Newspaper front pages showed Mr. Reagan daily denying that he had firm plans for raising taxes after election day and then explaining why ''revenue enhancement'' would be ''tax reform,'' not tax raising.
This follows the week in which headline attention was largely focused on what the Democrats had done by nominating a woman for the vice-presidency. That produced two weeks when the Democrats had the advantage in favorable front-page attention, leaving the Republicans trying to catch up. They advertised their many, and often distinguished, women in less conspicuous positions and tried to explain away the tax affair with less than full success.
The fact is that the Democrats have shown imagination and vigor during the open stages of the campaign and have actually taken the rhetorical initiative.
Of course the rational appraisal of the American political situation for 1984 is that the outcome will largely be determined by the state of the economy on the eve of election day. Present prospects are for continued buoyancy in the economy.
But Mr. Reagan is in a new kind of situation, and it remains to be seen whether he can do as well defending his own four-year record in the White House as he did when attacking the Carter administration's four-year record.
Reagan and his staff are said by observers around the White House to have gone into this campaign in a high mood of confidence, based on how smoothly and successfully Reagan performed in 1980. Do they by any chance underestimate the fact that it is easier to attack than to defend a White House record?
Looking back at the 1980 campaign, the wonder really is that Jimmy Carter did as well as he did. As election day approached the hostages were still in prisons in Tehran. The attempt to rescue them had been an embarrassing failure which seemed to expose a general state of unreadiness in the armed forces. Most important of all, the economy was still dominated by the decline and the closing down of the old smokestack industries.
It was an accident of history, not something for which Mr. Carter could be held responsible, that in 1980 the new industries had not yet begun to take up the slack from the decline of the old. Nor is Reagan responsible for the fact that during his four years in office the new industries came on line and did take up a lot of the slack from the decline of the old.
But politics is not concerned with the underlying cause of changes in the economy or the general welfare of peoples. Politics is concerned with the condition, not the cause of the condition. The general condition of the United States in 1980 was poor, so poor that Carter himself had complained of a ''national malaise.''
The 1980 campaign was a downhill run for the attacker. It was easy for Reagan to keep his cool and enjoy the psychological advantage.
It is different now. True, Mr. Reagan is not in as unfavorable a position as Mr. Carter was in 1980. The intervention in Lebanon was a fiasco. But it occurred far enough in advance of election day to be largely forgotten now. And then there was the easy win in Grenada to balance off against it. The economy has improved steadily over the past four years.
But still there is a Reagan record with flaws, and Mr. Reagan has to stand on it. Mr. Mondale is free to attack at will wherever he thinks he sees a vulnerable spot.
What we are about to find out, therefore, is whether Reagan will be as skillful on the defensive as he was four years ago on the offensive. Last week he showed signs of irritation over the tax issue. He seemed to be a little rattled. It may be an interesting campaign after all.