Sen. Edward M. Kennedy (D-Mass.) took his presidential campaign through the heart of the country today in search of new format for making his case against President Carter, and at a bright new senior citizens' center here to met a perfect format in the person of Silvester Bonnis.

Bonnis, 72, a retired factory workers, was among 120 older Chicagoans who heard Kennedy make a quiet campaign speech this morning on a favorite topic: the need for a comprehensive federally financed health-care plan.

Kennedy said Carter had abandoned senior citizens by refusing to fight for health plans to help them bear the cost of treatment and medicine.

After his speech, Kennedy strolled into the room to talk to his audience, but then, suddenly, he was making his way back to the podium, towing Bonnis, a white-haired man with a black cane.

Prodded by Kennedy, Bonnis stepped shyly to the microphone and explained what was up. "The senator just asked to me say what I just told him," he said. "I told him I have to pay $60 per month for prescriptions to stay alive. And I've got to live on a pension."

Kennedy prodded again -- "Pour it on, Silvester," he said -- and Bonnis added the kicker."I worked hard for 47 years, and if I ever had to go in the hospital, I'll lose everything I saved."

For Kennedy, it could not have been better. Bonnis, a genuine human infusion into the often artificial world of political campaigning, was great television. And for Kennedy that meant great politics.

The discovery of Bonnis was apparently serendipitous. But it fit neatly into the purpose of this first week of Kennedy's race for the White House.

In an impromptu news conference on his campaign flight this afternoon, Kennedy said he wants to "explore new formats over the next few weeks" as his campaign takes shape.

Twirling a long cigar and mugging for his sister, Jean Smith, who was taking snapshots all the while, Kennedy said he thinks the theme of his campaign is clear. "It's the failure of leadership," he said.

What is not clear, he went on, is the best way for him to get that theme across.

"I just think it doesn't always have to be the big rally," he said. "That fellow Silvester this morning said it a lot better than I could have, and if you could do that a lot of places you could really get the point across."

But he rejected the suggestion that he might try to find a new format to help explain again what happened at Chappaquiddick.

Asked if he would have anything more to say on that incident, he answered curly, "like what? There's nothing more to say. If there were, all this [campaigning] wouldn't make any sense."

Kennedy tried out another format later today in a meeting with 1,100 students at the University of Oklahoma at Norman. He took a series of opinion surveys by asking students to vote on issues by show of hands.

This was a technique frequently used by Kennedy's late brother Robert in the 1968 presiential campaign. The students indicated that they were strongly against further development of nuclear power, a position that is close to Kennedy's own. But they came out clearly against approval of SALT, to which Kennedy remarked, "I'm getting a picture down here I might not like."

Kennedy's reception from the students was mixed until the very end, when he brought down the house with his definition of presidential leadership: "I want a president who's going to take a stand on issues, go to the American people and say this is what we need, this is what we're going to fight for, and then go ask the Congress to say yea or nay on these issues."

The Kennedy camp added a new member to its press staff this week, Martha Angle, a former reporter for The Washington Star who has been writing a syndicated newspaper column with her husband, Robert Walters.