It is the 27th day of the crisis, and Hodding Carter III steps up to the podium, framed by a wall-sized map of the world. He stands very straight, thrusts his chin forward and stares out at 70 reporters, notebooks poised. He is bathed in the blinding light of six television cameras.

It wasn't always like this. The State Department's daily briefing, a ritual that occurs exactly at noon, was often a sleepy affair, rather like a friendly gathering in a club. And Carter was a man hardly known to the American public, the anonymous spokesman for a large bureaucracy.

Now this Mississippi-born former newspaperman who calls himself "a conveyor belt for policy" has had a prime-time television audience of 70 million for three weeks running. As the administration's voice on Iran, he is the one who must answer the daily torrent of questions and help soothe the passions of an aroused nation.

The pressures are extreme. "I have to be careful not to get carried away by the sound of my own voice and make policy by mistake," he said in an interview. "Every little thing you say is puzzled over to see if it means something."

"I'm afraid I could send a signal through a careless phrase which could have immediate consequences. A radio report could flash to a country and a mob could be on the streets before we had a chance to explain," he said.

Watching Hodding Carter perform at the daily briefing, it seems unlikely.

Dressed in a State Department uniform -- striped shirt, striped tie, navy blue suit -- he deftly sidestops the minefields. The dialogue of diplomacy is esoteric, nitpicking, even in these dramatic times. Carter is a master of elegant evasion: polite, controlled, never telling more than he intends.

"You slipped off the point," complained a reporter questioning him last week on the Soviet role in Iran. Carter retorted: "I hope so."

And, at another briefing, Carter was deliberately ambiguous as to whether U.S. Charge d'Affaires Bruce Laingen and two other diplomats are being confined in the Iranian foreign ministry. "I think that they have a strong sense that where they are is where they should be and must be and will be," he said.

While hardly a national figure, Carter had a brush with fame once before. At the 1972 Democratic National Convention he was nominated for vice president, drawing some 500 signatures before withdrawing in favor of Thomas F. Eagleton (D-Mo.).

Long befoe that, however, the name Hodding Carter had made a mark on southern politics. His father, Hodding Carter Jr., founded The Delta Democrat-Times, a newspaper in Greenville, Miss. "Big Hod" was known for his courageous support of civil rights, winning a Pulitzer prize for editorial writing on race in 1946.

Young Hodding went to Princeton University, wanting to be a foreign service officer. But a stint as a cub reporter in New Orleans convinced him to return to Greenville. He worked there 17 years, becoming editor and publisher of his father's paper and a leader of the Mississippi civil rights movement.

In 1968, Carter and other civil rights activists, calling themselves the Loyal Democrats of Mississippi, unseated the all-white state delegation to the Democratic National Convention after a bloody credentials fight. N 1972, the Loyal Democrats again won the credentials fight against segregationists. By that time, Hodding Carter Iii was co-chairman of the credentials committee.

He became a supporter of Jimmy Carter (no relation) and, at the beginning of the new administration, was named assistant secretary of state for public affairs.

One of the people who originally persuaded him to support Carter was Patricia Derian, an old friend from Mississippi and "a do-good liberal integrationist like I was." Last year, after Carter's divorce from his first wife, he married Derian, now assistant secretary of state for human rights and refugee affairs.

Away from the combat of the daily briefing, Carter, 44, is witty, charming and as easygoing as his southern drawl suggests. Vetern State Department reporters say that with virtually no knowledge of foreign policy, he quickly mastered the subject. He is a good source, with consistent access to Secretary of State Cyrus R. Vance.

"In my 20 years as a reporter, he's the best spokesman I've seen," said The Boston Globe's William Beecher. "He's intelligent, articulate. He tries to be forthcoming. I'm embarrassed to be so praiseful, but he's one of the few shining lights on the Washington scene."

Several reporters, however, criticized Carter's staff. "As a spokesman, Hodding is excellent," said NBC's Richard Valeriani. "But he's a disaster as an administrator. The press operation is very amateurish. Press releases have the facts wrong. It's a sloppy operation."

Valeriani and other TV reporters have more access than ever, however, since Carter was the first spokesman to allow cameras to film the daily briefings, a change which, incidentally, brought him into the limelight.

One reporter, remarking on Carter's new media celebrity status, said, "Now every time I go out to dinner, people ask me if I know Hodding Carter."

In an interview, in his carpeted, comfortably furnished office, Carter shrugs off his new fame. "It's ironic," he said. "It's ridiculous -- I really ought to endorse dog food. It's the Andy Warhol phenomenon. My 15 minutes will be up soon."

He jumps up to show a reporter some of his fan mail. He is expecially delighted with the neatly typed postcard from a man in Baldwin Park, Calif.

"I'm getting sick of looking at your ugly Commie face on television and hearing your pious reasons why we can't do anything about the events in Iran," it reads.

"Joseph McCarthy was right. The State Department is full of traitors in striped pants'"

:Carter looks down at his pin-striped suit and laughs. "Oh hell," he says.

"I think I'm fairly stiff on the tube. I hope that's the impression. If you come across as flip, then people wonder whether we are thinking seriously about policy."

Carter says he has no desire to exploit his newfound fame, although friends have long speculated that he might go into politics. He plans to leave his job after 1980, as does Vance, regardless of whether the president wins another term.

"This has been a fascinating job, but four years is long enough," he said. "This is not a substantive job. You are transmitting other people's policy. I spent 17 years as an editorial writer, telling the world what to do. This hasn't been an easy change of gears."