Cotter Smith, 33, had four days to come up with his portrayal of Robert F. Kennedy, to be revealed tonight in the first half of the four-hour television drama "Blood Feud."

"After a month of being interviewed, they called on a Friday to say I had the part--and that shooting started Monday. I worked for 86 hours on an interpretation, and the first scene Monday morning called for me to just walk down an alley. 'Wait!' I said. 'I have no idea how Robert Kennedy walks!' "

"Blood Feud," a $4 million production, dramatizes the public skirmishes between Jimmy Hoffa and Robert Kennedy--the one on his way to the presidency of the Teamsters union, and the other to becoming attorney general under his brother, John Kennedy. The two had seemingly opposite styles and different backgrounds, but are seen here as tough fellows who realize, as Smith puts it, "there isn't room enough in the town of Washington for both of them."

The ads say "introducing Cotter Smith," and indeed, although he has had only small television roles before, today he could be teetering on the brink of nonanonymity.

Cotter Smith's father is federal Judge Thomas Lewis Smith, currently a resident of the Watergate, and at first he didn't think acting was such a hot idea, his son concedes. "He wondered about the job security." After all, Cotter had gone to St. Thomas the Apostle School in the District, Landon School for Boys in Bethesda, Lawrenceville school in New Jersey and Trinity College, in Hartford, Conn., where he was an English major.

But when Cotter Smith was put up for the role of RFK, Judge Smith's "big, competitive Irish-Catholic family where the dinner table conversation was politics" was seen as a wonderful foundation.

Actually, what got Cotter Smith noticed as a short-notice Bobby Kennedy was his role as a racist southern lieutenant in a Negro Ensemble Company production of "A Soldier's Play" at the Mark Taper Forum in Los Angeles. He is a member of the company, complete with a diploma naming him an "honorary Negro, entitled to all the disadvantages thereto." The part of the lieutenant called for a heavy southern accent. "How they saw RFK in that role, I still don't know," he said.

He started acting here after a teaching job at the Maret school fell through. He studied under Joy Zinoman at the Studio Theater, with Tony Abeson, and performed at the old Washington Theater Lab, New Playwrights' and ASTA theaters, learning his trade.

Smith, who knew nothing about working in front of cameras, was pretty impressed with Robert Blake, who, eschewing the tics of his "Baretta" character, turns in a bully performance as Jimmy Hoffa.

"Blake is really a professional, and he's cured himself of all his personal problems," Smith said. "He told me, 'Listen kid, we've got to be good. There's no fistfights in this one, no car chases, just a lot of talk.' I realized that he was right on both counts--we had to be good, and boy, was I a kid."

When Smith thanked director Robert Newell for giving him the RFK part, Newell replied: "Don't thank me. You know better than anybody what a nobody you really are."

He was impressed with the skill that the vetaran actor Jose Ferrer brought to a very small role as attorney Edward Bennett Williams, who represented Hoffa. "Before we were going to shoot his scene, Jose Ferrer took me aside and said, 'Would you mind going to a corner and running through this with me?' What was I going to say? 'No, Jose, don't bother me now?' The truth is, I felt like asking him for his autograph. But he had worked so hard on those few lines, I really learned something."

In his crash study of Robert Kennedy film footage, Smith discovered that "the classic Bobby Kennedy pose is sort of looking down. He had a rumpled look, white shirt with the collar unbuttoned, sleeves rolled up just above the elbow." To suggest the Kennedy teeth, Smith drew up his upper lip some, but no attempt was made to alter his features with makeup or prostheses. "As for the accent, I wanted to be dead sure I didn't mimic him. If they'd wanted Rich Little, they'd have got him."

He didn't ask members of the Kennedy family for advice--"I don't think that would be fair." But he was interviewed by Maria Shriver, who works for P.M. Magazine. "We were talking about 'Uncle Bobby,' and I said to her, 'Is this strange for you?' She looked at me and said, 'Well, since you don't look anything at all like him--no.' "

Later, Ethel Kennedy complained that the program was unfair to her late husband, and Jimmy Hoffa Jr. said it made his father look like "a thug."

None of that is bad for a young actor seeking to be noticed. There are dangers in playing an American cultural hero the first time out (history can be a hard act to follow), but Smith has reason to believe his career is now launched. Meantime he's living out of a suitcase--he has no apartment and is staying with friends in Los Angeles--ready to go wherever work leads him.

"Sure, you're a bum and a gypsy in this business," he said, preparing to go off to the Watergate for dinner with his father. "But we'll see what happens. It's funny what gives you a start. Robert Blake told me how he got his first big break. He was one of the original 'Little Rascals,' you know. He told me he got the job because, at the age of 3, he could say 'confidentially, I think it stinks' better than any of the other kids."