AT MIDTERM, THE PRESIDENT is in the grip of nostalgia.

Things are not the way they used to be.

Even the Depressions are not the same.

The simplicities and verities he espouses, and which his party so lately recently embraced, are under attack.

"Where Is the Rest of Me?" was the title he gave his autobiography. Now he seems to be asking, "Where Are the Rest of You?" as he heads into the third year of "splendid misery," Jefferson's term for the presidency. For him it has been more splendid than miserable. It has left few marks on his face.

At his first news conference of 1983, the recurring, underlying theme was plaintive.

The world has changed. He, of course, has not. He retains his utter, childlike faith in the free economy and his own theories. He retains the optimism that made a poor boy from Illinois a Hollywood star.

He thinks there are plenty of jobs out there, if people would go out and hustle. He reads the "help wanted" ads and they are becoming "really desperate" in companies that are advertising for people with certain skills.

Most people apply the word "desperate" to people, rather than ads. But not Ronald Reagan.

He spoke at his press conference of the unemployed, who are not working or paying taxes, as being "a cost item to the government."

He has for the past month been offering solutions which sound willfully mindless. At Christmas, he suggested that the employed try to make the unemployed happy. He did not say how. He never does. When he talks of voluntarism, he speaks of such community enterprises as "barn-raisings" which are of another era. His unemployment solution: "if each business hired one person." This from a man who has spent his life inveighing against hiring-as-a-favor by the federal government.

He obviously expected little response to his inane initiative, and that is what he got, although he said a bit defensively at his press conference that some businessmen had said "it had caused them to think" -- about what, he did not say.

As Reaganomics unfolds amid ruin and rebellion, he is just adlibbing. All will be cured -- failures, foreclosures, $200 billion deficits -- when the recovery he sees just around the corner -- "all the indicators are there" -- sets in.

He is almost alone in his view. Even his best buddy, Sen. Paul Laxalt (R-Nev.) deserted him on the ramparts. He came out of the White House, rolled down the window of his car, and spoke of "terrifying" deficits and the possibility that "we're going to have to look at deferral of some military spending."

Twice, he was asked about Laxalt's defection. He did not answer. The public refusal of his favorite senator to wish upon a star may not have been in the script.

The other day, when press secretary Larry Speakes was asked who in the presidential circle wants to stay the course with the president on the military budget, he laughed, pointed back over his shoulder, and said, "Cap" -- meaning, of course, Defense Secretary Casper Weinberger.

The president thinks the clamor to treat the defense budget like any other comes from the "constant (media) drumbeat that somehow there is a needless extravagance and that we're overboard on this particular subject."

He misses the military-industrial complex of yesteryear.

"You have to remember, we don't have the military-industrial complex that we once had, when President Eisenhower spoke about it."

In Eisenhower's famous farewell speech, he said, "We must guard against the acquisition of unwarranted influence . . . by the military-industrial complex. The potential for the disastrous rise of misplaced power exists and will persist." It was a warning, not a tribute. One man's menace, it seems, is another man's loss.

And a question about the recent Miami riots -- could the riots be caused partly by economic hardship? would they be repeated? -- brought on this arresting bit of nostalgia.

"In the Great Depression nothing like that ever took place, when the situation was much worse and there was no unemployment insurance . . . not even any welfare programs."

People took things better in the good old days.

He seemed most at home recently at an occasion as anachronistic as his thinking. He was on the deck of the newly recommissioned battleship, New Jersey. Surrounded by the Navy blue and gold, he had "a strange feeling" that he was back on the set filming "Hellcats of the Navy." It was a picture based on the "great, victorious operation of the Navy in World War II" he said.

Some people who watch the president as he rambles down Memory Lane are begining to wonder if it will not lead him straight into irrelevancy.