At the back of the green-and-white circus tent, pitched in a quiet park beside the Rock River, Henry Sadewatter surveyed the crowd and said sadly:

"Strangely enough, he doesn't have the full support of Rockford. The reason for that is you have too many dyed-in-the-wood Republicans and Democrats.

"I'd think the modern thinker would have to go with him," Sadewatter, an electrical contractor, continued. "Not because he's right all the time, but because he believes in the things people in Rockford believe in."

Sadewatter was talking about John B. Anderson, who has represented this city in Congress the last 20 years.

Rockford, a northern Illinois city of 125,000, staged a welcome-home rally for Anderson taody. It was his first public appearance here since he broke ranks with the Republican Party and launched an independent presidential campaign April 24.

Mayor Robert McGaw, a Carter delegate to the Democratic National Convention, gushed over the congressman, calling him "the most famous citizen we've ever had, the greatest Rockfordian in Rockford history."

He praised Anderson's effectiveness in Congress. "We can't afford to lose his voice in Washington," the mayor said. "So let's keep him there in a more strategic position." An Andeson presidency, the mayor added, "would be the best thing that ever happened to this city and you know it."

Yet police estimated only 1,000 people in Illinois' second largest city had shown up to welcome home its most famous citizen. Organizers attributed this to a light morning drizzle and the Labor Day weekend.

But there also is an ambivalent sort of chemistry at work about the Anderson candidacy. "A lot of people here are just surprised he's still in it," explained one young man.

Those who did greet Anderson were loud and enthusiastic. They cheered wildly when Anderson was introduced as "the next president of the United States," and a "silver-haired orator, with a golden tongue, a 17-jewel mind and a brass backbone."

And they cheered again when Anderson's vice presidential running mate, Patrick J. Lucey, the former Wisconsin governor, illustrated one of the campaign's attacks on Republican and Democratic tickets with the answer a University of Wisconsin student gave to the question, "What is the difference between Jimmy Carter and Ronald Reagan?"

Lucey quoted the student as saying "Carter is liable to get us into war by accident. But Reagan would do it on purpose."

Anderson picked up the same theme later, attacking the Carter administration's recent warnings to the Soviets about the possibility of nuclear war.

"In his effort to show his machismo he [Carter] is telling the American people that nuclear war is winnable," Anderson said.

"This madness has got to stop."

Earlier in the day, Anderson, his wife, Keke, and daughters, Karen and Susan accompanied his 94-year-old father, Albin Anderson, to the First Free Evangelical Church which the senior Anderson joined when he arrived here as an immigrant from Westergotland, Sweden, in 1907.

The senior Anderson operated a small grocery on the east, predominantly Swedish, side of the city. The church, Anderson has written, was one of the most powerful influences on his life.

It was in a tent meeting sponsored by the church that Anderson, at age 9, became a-born-again Christian, he has said.

"There under the canvas of what had once been the 'Big Top' of one of those innumerable little circuses that toured the country but have now become as sanctified as any great cathedral, I fell on my knees and beseeched God's mercy," he has written. ". . . When my turn to testify came, I simply said, 'I know that He knows that I know Him.'"

Anderson has maintained a relationship with the church throughout his life, but it has recently moved to a new multimillion-dollar suburban sanctuary and this was his first visit since that move. His support of legalized abortion in the campaign angered some church members, and a district body of the Evangelical Church asked him to reconsider his position just before the Illinois Republican primary in March.

Today the pastor, Lareau Lindquist, called the district body's action an "unfortunate-type incident." And he gave the Anderson family a warm greeting from the pulpit saying, "I assure you our prayers are with you. We pray for your strength, forbearance and wisdom."

The stop in Rockford was to have been the start of a whistle-stop trin tour across the Midwest by Anderson. But the train trip was dropped Thursday when the campaign decided it would be too expensive and it encountered logistic difficulties.