President Reagan, who loves to invoke the names and aspirations of long-dead Democrats, slipped up once this month in Texas and quoted a Republican president, Dwight D. Eisenhower.

Perhaps the mistake can be traced to the strains Reagan faced that day, when he was being pressed for comments on the indictment of his secretary of labor, Raymond J. Donovan, and for an explanation of lax security at the bombed U.S. Embassy annex in Beirut.

But the Eisenhower reference -- "to be strong nationally is not a sin, it is a necessity" -- was at least appropriate. Much of the time in his reelection campaign, Reagan has behaved as Ike did in his 1956 peace-and-prosperity reelection campaign when he all but ignored his opponent and delivered a message satirized by the Democrats as, "You never had it so good." Had it not been for the presidential debates, it is doubtful if Reagan would have raised an issue or even mentioned the Democratic nominee, Walter F. Mondale, by name.

Mondale seems locked in his own time warp. It is as if he had left the country during the Reagan recession and failed to observe on his return that an economic recovery had occurred.

Despite his performance in the first debate, Mondale's refusal to recognize that many middle-class Americans now consider themselves "better off" has cast doubts on the credibility of his campaign. He has compounded this problem by endorsing various conspiracy theories about the second-term intentions of Reagan, who Mondale seems to think has "secret plans" for doing everything from raising taxes to invading Nicaragua.

Reagan intimates whisper about a sadder truth, one they hope doesn't become too evident before Election Day. Their concern is not that Reagan is richly endowed with secret plans but that he might not have any plans. In this view, the shadowy and unfocused character of the Reagan campaign is perfectly synchronized with the prospects for Reagan's governance during the next four years.

Oh sure, Reagan still believes in the bromides and would push for tax simplification, a bipartisan idea whose time has come. He would submit a big defense budget, which even a Republican-controlled Senate is likely to trim, and he would call for domestic spending cuts and such marginal irrelevancies as the line-item veto.

But those near the president find no sign that he has the old fire in the belly that for the past two decades made him the most formidable figure in national politics.

In 1980, the Reagan goals of "restoring America's defenses," reducing income tax burdens and "giving government back to the people" had attributes of a crusade. Even in 1982, "Stay the course" had a defiant ring to it. This time, the battle cry could well be "Let's hold on."

One touchstone of the Reagan approach to a second term is his view that the federal deficit will take care of itself through economic growth. As a campaign tactic, that statement is understandable. What is appalling is that Reagan believes this nonsense, even though it contradicts his economists and the doctrines he has been preaching for decades.

It is in the context of Reagan's passivity that his recent slip-ups should be examined. On the campaign trail last week, most of these slip-ups were minor -- a question he couldn't recall that had been asked 30 seconds earlier, a misremembering of how he acquired his boyhood nickname of "Dutch," a re-reading of a speech passage after he had lost his place.

None of these mistakes came close to justifying the ugly little posters showing up at the fringes of campaign rallies that crudely contend that Reagan is too old to function as president. But they do provide glimpses of a man who, seen up close, is slowing down and has nothing new to say.

This would not be particularly awful if Reagan were more engaged, if he had Cabinet members who were young and energetic, or if he were still consumed with a desire to make big changes in government.

But in the absence of any of these, the chant of "four more years" has a hollow sound of vagueness and drift. The real concern about Reagan is not that he will unleash a secret plan. It is that what we are seeing in this campaign is just what we can expect.

Reaganism of the Week: Forgetting a question about federal aid to colleges at the University of Alabama last Monday, the president said, "I got so carried away, maybe I forgot about your question there. Help me out."