Thomas Hearns, the Detroit "Hit Man" scheduled to meet Sugar Ray Leonard in a championship bout this fall, came into his opponent's back yard yesterday seeking to establish his own identity and charm the nation's capital.

At a luncheon held at the Rayburn Building by Rep. John Conyers Jr. (D-Mich.), the soft-spoken World Boxing Association welterweight champion was greeted enthusiastically by House Speaker Thomas P. (Tip) O'Neill (D-Mass.), Mayor Marion Barry and others.

"Tommy, you certainly can hit like a mule," O'Neill told him. "Listen, son, you're a champ. In everything I've seen you in, you have it. Always act like a champ."

Later, the Tommy Heans Show continued in a crowd-pleasing eight-round exhibition at the District of Columbia Correctional Facility at Lorton.

Throughout the afternoon, at both the luncheon and at Lorton, the absent Leonard, the World Boxing Council welter-weight champ, was present in the questions.

Leonard and Hearns will be on the same card in the Houston Astrodome June 25, Leonard to challenge WBA junior middleweight champion Ayub Kalule and Hearns to defend his title against unranked welterweight Pablo Baez.

Hearns bristled when someon asked if he would be a preliminary fighter on the card. "I don't think I should be under Leonard," he shot back. And Emanuel Steward, Hearn's trainer and manager, said Hearns would have withdrawn from the fight if he hadn't received equal billing with Leonard.

"I don't dislike Ray. I have nothing against Ray as a fighter," Hearns said. What Hearns does have against Leonard is his refusal to fight him over the years.

"I was coming up and Ray didn't want to give me that chance," he said at the luncheon. "I think everybody should be given a chance. When you're a champion you have to take on all comers."

His goal, he said, is to be a four-time champion "from welterweight to light heavyweight" in three years.

He'll start working on that goal by trying to become the undisputed welterweight champion Sept. 16, when he and Leonard meet at a site still to be determined. Hearns, undefeated in 31 bouts with 29 knockouts, is expected to receive $5 million and Leonard reportedly has been promised $8 million. They will split a portion of any revenues in excess of $21 million.

At the end of the luncheon, most of the entourage went to the Lorton gymnasium. Hearns, a volunteer auxillary policeman in Detroit, said he wanted to do the exhibition to encourage the men on the Lorton boxing team.

"The lights give the men something to hold to," Steward said. "They can always remember what it was like to get inside the ring with the welterweight champion of the world."

The prison gymnasium was jumping with the sounds of Thomas Browne's album, "Let's Dance." More than 300 inmates sat on bleachers, stood along the catwalk or crowded in at ringside, challenging Hearns to show them how he had earned the title "Hit Man."

They applauded when the 22-year-old fighter arrived. The 6-foot-2 1/2, 147-pound boxer was wearing a crimson shirt and shorts outfit with a blue stripe and stars riding down the arms and the sides of the legs. On the back, in white letters, were the words "World Champion Tommy Hearns."

Immediately, the taunting began.

"What you got to say about Sugar Ray?" the inmates yelled.

"Me, I like champ. I'm not prejudiced," one inmate told another speaking of Hearns.

"I think Joe Sweat will take him," his friend responded.

"Hey, man, this is only sparring," another inmate chimed in. "This is not no championship."

But it was like a championship fight for Hearns' four opponents: Hollingsworth Bey, Stanley (Jack Johnson) Vance, Larry (Golden Boy) Pringle and Joe (Marine Boy) Sweat. Each spent two rounds in the ring.

"It's a pleasure sparring with the champ.It gives me some good experience," said Pringle, who has been fighting since he was 9 years old.

Pringle has been at Lorton five years and on the boxing team four. "Everything they brought down here I've been putting 'em down for a TKO," he said.

Sweat said he joined the Lorton boxing team four years ago "for an opportunity like this." Sweat has been imprisoned six years, and says he never fought on the outside.

Sweat was the first inmate Hearns took on yesterday.Hearns started out the exhibition flat-footed, plowing in with a flickering left. At one point, Hearns backed into the rope. "Don't go for that rope-a-dope," the crowd yelled at Sweat. Unexpectedly, Hearns grabbed Sweat, swung him around into the ropes, and began to pound him as the crowd looked on in amazement.

Against Pringle, Hearns came out dancing, as he did against the remaining two fighters, chasing his opponents and thrilling the crowd with jabs and feints that threw the fighters off balance and nearly had some of them kissing the canvas.

Conyers refereed two of the rounds.

The crowd applauded; stood up and cheered; laughed and called to the "Hit Man" with newfound respect.

"He's spectacular!" explained inmate Caswell Flemings, 23. "I really like him."

But Flemmings, like most of Hearns' fans, somehow could not see him defeating Leonard.

"Whereas he's spectacular, Ray Leonard is devastating," Flemings said.