"Big Mo" is gone from the George Bush campaign. So is Asterisk One, its old airplane.

The candidate has lost weight. He is back wearing the horn rimmed glasses he set aside on the advice of his political experts last summer. His face is etched with deep lines of fatigue.

The candidate-jogger is dogtired. So tired he is taking a second day off Sunday while the 69-year-old man he is running against continues campaigning across the Midwest heartland.

There is an air of desperation about the Bush campaign. A reporter joins the Bush traveling entourage and is promptly asked, "You covering Illinois or writing a Bush obituary?"

The candidate is understandably frustrated. The Illinois Republican primary next Tuesday was where he had hoped to deal a severe blow to frontrunner Ronald Reagan. But events have spun out of his control.

Yet at the same time, there is alarm in the Reagan camp despite his four straight primary victories in the South. The political director of his Illinois campaign said today that Reagan may lose here Tuesday to Rep. John Anderson if the turnout exceeds 1 million.

"The election hinges on the turnout, and I'm worried about it," said Don Totten, and Illinois state legislator who directs the Reagan campaign here and in Michigan. "My prediction is that the turnout will be slightly over 1 million because of heavy Democratic crossover votes, and this means that Anderson could take it."

Reagan himself, campaigning strenuously from early morning until far into the night, seemed a bit more optimistic.

The former California governor said he had "a good feeling" about winning the downstate vote, but didn't know how he would do in Chicago and its suburbs. Asked whether he had the feeling he would win the primary outright, Reagan told a Midway Airport press conference: "I'd rather described it as a feeling of hope."

Bush, meanwhile, feels he has to do something. Now trailing Anderson in the polls, he has awkardly gone on the offensive against him.

The candidate who had leaflets printed talking about his momentum -- or "Big Mo" as he called it -- now says he will talk only about issues. As he sees it, the chief "issue" is Anderson.

But the pressure is really on Bush after disappointing finishes in recent primaries. He is airing a hard-hitting television commercial that accuses Anderson of signing a fund-raising letter to help Democrats like Sens. Frank Church, George McGovern and Birch Bayh and suggests that Anderson really isn't a Republican.

Last night when he addresed McLean County's Lincoln Day dinner here, Bush began saying, "I want to spell out some differences between me and John Anderson. He's the front-runner in his home state, after all. The differences happen to be the same differences between me and Jimmy Carter.

"I don't believe in a 50-cent tax on gasoline like Anderson, for example," Bush said. "He calls it the 'Anderson difference.' I call it the Bush difference!"

Bush then launched into a 20-minute frontal assault on the Illinois congressman's stance on abortion, arms limitation, the equal rights amendment, military spending, draft registration, Social Security, the shah of Iran, and especially on Anderson's refusal to say whether he would support the next Republican presidential nominee.

"I will not send out a fund-raiser letter for Frank Church and Birch Bayh," he shouted. "These are the people I want defeated."

Bush received a polite, not overly enthusiastic reception from Republicans who had paid $25 a plate to attend the dinner. He clearly separated his positions from those of Anderson. But sample interviews in the crowd indicated that few people were Anderson supporters.

That's what makes the Illinois primary Tuesday so confusing. Anderson's appeal is not to traditional Republicans. It's to young people, Demorcrats and independents.

"There's a tremendous imponderable in this district," Rep. Edward Madigan said after Bush's speech. "That imponderable is seven universities. I understand that, in the last few days before registration closed, 2,000 students registered in Champaign County. And there could be only one reason for that. That Reason is John Anderson."

Anderson was greeted by one of the largest crowds of the campaign today when he visited Champaign County and his alma mater, the University of Illinois. About 1,900 students packed the university's old Memorial Auditorium; hundreds more spilled out onto the steps and grass. The crowd was wild about Anderson. It stomped, it shouted, it applauded endlessly. It greeted almost every remark of the snowy-haired congressman with thunderous applause.

Anderson, nearly overwhelmed, said he had returned to the campus for many homecomings. "But I tell you none has thrilled me like this reception today.

"Nothing could make me prouder than if next Tuesday the young people of this state send out a signal that they want John Anderson to be the next president of the United States."

In Peoria, Reagan was greeted enthusiastically at an airport hanger rally that featured balloons, a patriotic band and the presentation to Reagan of a 4-month-old piglet wearing a red, white and blue ribbon.

The candidate gingerly patted the pig, then returned it to owners Kenneth and Donna Zimmerman. They said they would keep it on their 400-acre farm near Morton and give it back to Reagan after he became president so that the pig could graze on the White House Lawn.

A poll published in Sunday's editions of the Chicago Sun-Times demonstrated that Reagan and Bush have reasons for their concern. It showed Anderson running stronger against Jimmy Carter than any other GOP Candidate in November. The poll of 1,001 Illinois voters showed 49 percent favored Carter and 42 percent Anderson. Carter did far better against other Republicans, topping Reagan by 60 to 34 percent and Bush by 42 to 36 percent.