The president and I took a trip to Illinois this week. Actually, we didn't go together. He flew on Air Force One, and I went on one of the two jets paid for by the traveling press corps. In Illinois he went from place to place on a Marine helicopter, and I went on a press bus. In fact, I only saw him twice, both times from across big crowds in school gymnasiums. I would have seen him better on TV. Still, things look different in real life, or at least that aspect of real life that a media event presents to an on-site observer. You get a better feeling for the massive behind- the-scenes activities of the principal players in a presidential trip: the White House staff and the media.

The president, of course, has a critical but brief role to play. And the carefully selected crowds are an important backdrop. Basically, however, the scene plays out as a struggle between the horde of White House advance men, transportation and press aides, and the army of wire service, newspaper, television, radio and magazine reporters and the cameramen that follow the president in search of news.

The White House staff is trying to make sure the journalists get the story -- and only the story -- that they want them to get. The journalists share the staffers' wish for a spot on the front page or the evening news, but they are hoping for more lively material than the rehashed Reaganisms in the advance-speech texts handed out on the planes. So the more dutiful begin typing a first take, trying to make the material in the prepared texts sound like news. They know they will have to file something -- and they won't have much time to do it between dashing for buses and pushing through crowds.

For me, an observer of the observers, the flight is pleasant -- attentive stewardesses, a multi-course breakfast, candy bars to put in my pocket as I get off the plane. Then comes the endless wait for something to happen -- a mind-deadening, time-stretching experience that one seasoned reporter calls the "White House lobotomy." We sit in one of several press buses, chat with a roving White House aide and try to stay alert, waiting for Air Force One to land and the president and a small pool of media representatives to be helicoptered off to talk with a few students in a job-training center.

Finally we ride to the first "open press" event. This is a speech before the student body of the Bolingbrook High School in a predominately Republican, white, lower-middle-class suburb of Chicago.

More waiting. The president arrives with Illinois Gov. James Thompson and other Republican dignitaries (tumultuous cheering). The governor talks about the "extraordinary bond" between Reagan and the younger generation. He gives a baffling analysis of how the Democrats would increase deficits and thus burden young people's pocketbooks in the future. He introduces the president as "the man who wants to give the American dream back to you" (big cheer).

This warm-up turns out to be the day's major theme: a pitch to the youth of today in their capacity as the tax- haters of tomorrow. The students -- many of whom say they already hold jobs and resent payroll deductions -- respond to it.

The president looks tan and rosy-cheeked, but he stumbles over his opening remark: "It's great to be back in the -- or in the proud town of Bolingbrook." He slides into the rhythm of his prepared text, painting a picture of a ghastly past in which all the troubles of the last century seem concentrated in the four Carter years. He contrasts this Dark Age with a glorious future promised by his policies. His voice is soft and avuncular -- patronizing, actually -- as he talks about how "Coach Tax Hike" (Walter Mondale) and his "Tax Increase Team" want to burden the economy. His own "American Opportunity Team" wants to build a "land of prosperity, pride and hope." The applause is polite. The few black students, as well as a small number of white students, never stand, applaud, cheer or even smile.

After the speech, a few students ask carefully prepared questions that enable the president to accuse the previous administration of "unilateral disarmament," and reaffirm his commitment to Social Security and tidy national parks, while dancing around the issue of federal student aid. The president praises the students who give him a football jersey emblazoned with his nickname, "Dutch." He responds with another version of how he got the name. It's not the version he gives in his autobiography. The journalists dash for the restrooms and then to the buses.

A long bus ride -- during which the daily reporters update their copy -- brings us to the prosperous suburb of Glen Ellyn. A big Reagan-Bush rally has been organized in a vocational college gym. The crowd is already being led in chants of "Four More Years" and "Reagan, Reagan." Overhead signs say "Every 1 Wants Ron," "Women 4 Reagan" "Freedom Is Human Right No. 1." A lonely demonstrator, plastered with anti-nuclear signs, wearing a homemade gas mask and carrying a plastic skeleton, wanders silently through the crowd. No one pays any attention.

There are, as far as I can see, only two black people in this audience, a businessman and his son. The man turns out to be a Mondale supporter who came because his son wanted to see the president. Another reporter from The Post talks to various members of the audience. They cite lower inflation and a better economy as reasons for supporting Reagan. Some are in businesses hard-hit by the 1982 recession, but, like the president, they tend to think of the recession as part and parcel of the Carter years of "torpor, timidity and taxes."

In Glen Ellyn the president is off his stride. His voice is husky and faltering. A garbled joke about his age falls flat. He loses his place in the text and repeats a few lines before he recovers himself. Through my binoculars, he looks either tired or bored. But the faithful in the front ranks keep up the chants, applause and boos appropriate to the president's text. From my vantage point on a press stand high above the crowd, however, I notice that people in the middle and back of the huge audience are acting restless as well.

The president gets a second wind. He launches into a spirited attack on the errors and inconsistencies in Walter Mondale's record. The crowd responds with enthusiasm. The president asserts that, whereas his opponent supports controlled prices, "we decontrolled oil prices. It was one of the first things I did." Since it is an indisputable fact that Jimmy Carter began oil price decontrol, I check the advance text to see whether the president's mistake was ad-libbed. I'm surprised to find that it is part of the prepared text and wonder why it wasn't corrected.

While I'm watching the speech, the daily reporters are filing updated copy and hoping that their divided attention doesn't cause them to miss some important event. After much standing in line and running through muddy parking lots in the rain, we are back on the buses and, ultimately, back on the planes.

Members of the press corps amuse themselves in flight by rolling oranges down the aisle. Someone tries to roll a banana. Back at Andrews Air Force Base we get a handwritten update from the pool reporters on Air Force One. It informs us that Larry Speakes, the president's chief press spokesman, "passed out the president's schedule for Wednesday, announced there would be no 9:15 briefing tomorrow, and returned to the front of the plane."

Back home, I turn on the television news to see on the small screen the reality that I have witnessed during my 14 hours on the campaign trail. I want to know what I am supposed to have seen. The president, appearing larger than I ever saw him, is telling another joke about his age (this one falls flat, too) to a few students at the job-training center he was helicoptered to early that morning, the one we didn't get to visit.

Suddenly I feel as if I had missed the whole day.