The good news for Democrats is that Walter F. Mondale does, after all, have an instinct for the jugular. The bad news is that it seems to be for his own.

One day in his campaign is illustrative. In a few hours, he inadvertently made sure that the public's gravest doubts about his leadership capacity were reinforced; he revived the charges that he is a fainthearted -- and boring -- candidate.

What made Tuesday so baffling to the press corps that lumbered after him was that it came so soon after he had come on as "Fighting Fritz." On Sept. 25, he spoke at George Washington University and, to the satisfaction of at least his staff and the politicians who follow his movements with aching concern -- proved he could make an eloquent and hard-hitting attack on the popular incumbent.

Monday, in New Jersey, before a wildly responsive audience in a grubby old New Brunswick movie theater, he pressed harder. President Reagan, he implied, is neither in touch nor in charge. ". . . . He has not mastered what he must know to command his own government."

Among charges Mondale made was that Reagan is so uninformed about arms-control issues that he mistakenly thought that nuclear missiles could be recalled once they were fired.

On Tuesday morning, Mondale tried to call back a word he obviously thought would be radioactive and offend what is largely becoming the country's largest special-interest group, Americans who plan to vote for Reagan no matter what Mondale says about him.

The news media reported that Mondale had accused the president of being "incompetent." Consternation broke out in Mondale's camp. Mondale had broken his rule against "negative" campaigning. A Mondale campaign aide called NBC News to demand a retraction. Mondale denied at an airport news conference that he had called Reagan "incompetent," even though the sketch he drew of the president in his speech seemed to fit Webster's definition exactly: "without adequate ability, knowledge, fitness, failing to meet requirements."

"What I'm talking about is his management style," Mondale protested. "I did not talk about competence at all, did not intend to."

It was, at best, a Nixonian device to insist that you are not raising an issue when the words have just come out of your mouth. Richard M. Nixon said in the last desperate days of 1960, when it was his only hope, that he certainly would not bring religion into the campaign.

At his airport news conference, Mondale gave a lawyer's dusty answer about the indictment of Labor Secretary Raymond J. Donovan, a development that might have heartened another candidate in his situation, but that seemed only to make him more anxious. If he were president, he would fire Donovan but "for different reasons" than the indictment, he said. It was okay for Reagan to grant Donovan a leave of absence while he determines if the indictment is frivolous.

Mondale referred to "a tacky element" in this administration, then quickly erased it. He didn't want to be "pious" about it -- "there will always be violations of the public trust in any administration . . . . "

He said nothing about the need for Reagan to impose a higher standard than criminality in the selection and retention of Cabinet officers. His advisers clearly had warned him against Carter-like self-righteousness, which would be risky anyway in light of the blot on Mondale's copybook: his use of political action committee money in the campaign for the nomination is under investigation.

The Reagan reply on Donovan, history's first indicted Cabinet officer, shows how uninhibited a politician Mondale is up against. Reagan has no hangups, which is presumably one reason why the public dotes on him so.

In Texas, symbolically sheltered by the wing of Air Force One, the president preposterously spoke of the "lynch atmosphere" surrounding his persecuted appointees.

That is, of course, vintage Reagan. He takes off like Superman from whatever window ledge he has been forced out on, soars over leagues of disputed territory and lands so far off the mark that he leaves his critics speechless.

The words "strike that" never cross his lips. Mondale used them at his news conference when, still trying to call back "incompetent," he said that Reagan's lapse on irretrievable missiles was "a pretty grievous departure from reality."

Mondale flew from Washington to Little Rock, where he delivered a leaden speech to a wooden crowd of delegates to the National Rural Electrification Association. Granted that the agricultural economy is not a gripping subject -- Mondale's thoughts on "target-price mechanisms" sent several audience members off to dreamland -- but the candidate did not even try to get a rise out of the crowd.

His thoughts, it is said, are on the first debate, which he mentioned in passing.

Mondale has given Reagan no cause for alarm on the eve of the encounter. Reagan does not fear someone who does not go for his opponent's throat -- and acts, in the face of overwhelming evidence to the contrary, as if he had something to lose if he did.