Marvin Mandel, shadowed by his old friends and loyal assistants, marched up to the second floor of the State House this afternoon for the first time in 18 months, sat down in front of a battery of microphones and cameras, and for 30 minutes answered questions from the press in his uniquely vague style, never letting on whether he planned to resume power for the final five days of his second term.

"I am governor," said Mandel, in his moment of royal splendor. But he would not tell the 200 reporters, politicians and friends in the room whether he wanted to be governor in name or in function. "I see no need for any big rush about that. I think I'm going to think about what I'm going to do."

In the 21 hours since he learned that an appeals court in Richmond had overturned his conviction on political corruption charges, Mandel said, he had been too busy celebrating, too preoccupied with the euphoria of the moment, to consider the practical question that everyone wanted him to answer.

Blair Lee III, the acting governor for these months of Mandel's exile, a man as direct as Mandel is circumspect, told the press that he was as anxious as they were to find out Mandel's intentions -- and just as much in the dark.

"I had a brief meeting with Marvin to find out about my future and didn't get anywhere, so I came out here in the hope of getting some scraps of information," said Lee. "Obviously, he's not letting on, so I'm going to follow him around for the rest of the day." Two hours later, after another meeting with Mandel, Lee was still waiting for the word.

"Marvin," he said, "talked about everything -- except that which is relevant."

Indeed, on this day, it was hard to tell what was relevant and what was not. For Mandel, the time was unimportant and the sense of vindication unremitting. He seemed totally convinced that U.S. Attorney Russell T. Baker -- who has declined comment since the appeal court's decision -- would decide not to put him on trial for a third time. "Enough is enough," Mandel said.

It was a day of symbolic restoration, as well, for some of the aides who fell from power with Mandel's conviction and disgrace. They barged into the State House today with trench coats flowing and cigars blazing, shouting, "the Muldoons are back, the Muldoons are back."

"It's nice to be back," said Mandel's former legislative aide, Frank Harris, flipping a pack of quarters in the air.

"Yeah," said Michael Silver, Mandel's former deputy patronage secretary. "I got the nameplates in my pocket so we can put 'em back on our doors."

"Hey Ronnie," said former appointments secretary Maurice Wyatt to former deputy legislative counsel Ronald Schreiber, "you still selling tickets to Marvin's press conference?"

'It's like a class reunion," said Silver, as they burst into Wyatt's old second-floor office and posed for group pictures.

Some of the legislators at the State House joined in the merriment of the men who were known as Mandel's "roadrunners." Del. Paul Weisengoff showed off a new tie that had horses on it. "These are back in style again," he joked.

A central part of the case against Mandel involved his alleged attempt to get the legislature to override his veto of a switch of horse racing days from one track to another.

There were many other legislators, however, who expressed embarrassment about Mandel's return. When Del. Fred Rummage attempted to get the Prince George's delegation to pass a resolution congratulating Mandel, he could find only two other county delegates willing to vote for it.

"I thought I was elected as part of a new Maryland," said freshman Del. Thomas Mooney. "Now this..."

Added Del. Timothy Maloney: "Just because there were defects in the second trial is no reason to congratulate Mandel. Maybe it's reason to castigate the judge."

But even for some of the legislators who shared this view, the electricity of the moment was too much to keep them away from the reception room when Mandel held his press conference. Dozens of legislators -- Mandel friends and enemies -- stood in the back of the room and watched him perform.

Mandel was in a conciliatory mood during his appearance on the second floor, refusing to say harsh words about the politicians who ran against his image in the last election, even declining to criticize the men who prosecuted him. "Bitterness," said Mandel, "has never been a part of my makeup."

Attacked by Gov.-elect Harry R. Hughes during the September Democratic primary as the man who made people "ashamed to be Marylanders," Mandel has not been invited to attend Hughes' inauguration. Pressed on the matter by reporters today, Hughes said he would "have something to say on that later," but thought that the overturning of the conviction would have "no impact" on his own administration.

Mandel said he found no irony in the fact that his conviction was overturned only days before one of those politicians who ran against his image will be inaugurated.

"Well," said Blair Lee, who lost to Hughes in the primary, "If he [Mandel] doesn't [see irony], I do."

Mandel said that even if he decides to reassume power he will not move back to the governor's mansion.

Mandel said that next Wednesday, after his term officially ends, he would like to go back to practicing law. He even left the door open for a possible return to elected politics, saying, "I'm shaking my head only because I'm not sure," when asked whether he might run for a U.S. Senate seat in 1980.

The State House, with its excitement and symbolic gestures, contrasted markedly with the quiet, business-like federal courthouse in Baltimore, home of the U.S. attorney's office that proseccted Mandel. Inside, Baker piled up telephone messages, declining to respond to queries. His secretary issued a brusque "no comment today" when asked questions.

Prosecutors have 14 days from the time the opinion was issued at 4:40 p.m. Thursday to ask the appeals court in Richmond to rehear the case. If they decide not to seek a rehearing, lawyers say, prosecutors can take their appeal to the U.S. Supreme Court, retry the case in U.S. District Court or simply drop the charges.

Deputy Attorney General George Nilson said Mandel would now be entitled to a full pension and back pay for the months from which he was suspended from office -- amounting to more than $30,000.

At an early morning press conference at his Baltimore law office, Mandel's chief defense lawyer, Arnold M. Weiner, said trying the case a third time -- the first ended in a mistrial Dec. 7, 1976 -- would amount to "triple jeopardy."

"The public should be spared the expense and drain of federal resources which would be required if the preoccupation with this case were to continue," he said. "This case involves events which took place 10 years ago, during an administration which is almost over."

The costs of trying Mandel twice so far have run into millions of dollars. The defense alone has cost $1 million, and the prosecution has spent far more in more than five years of investigations and trials, according to lawyers involved in the case.

With four days remaining until his second term expires, Mandel can regain the full privileges and powers of his office simply by writing a letter. In the letter, Mandel would have to revoke a June 1977 executive order that designated Lee as acting governor while Mandel went through the twin strains of recovering from a mild stoke and beginning his second trial.

"I haven't even thought about it or discussed it," Mandel said in typically cryptic style shortly after walking into the governor's reception room for his first press conference there in 18 months.

As the day progressed, Lee remained among the curious spectators walking through the State House halls saying he had no idea of Mandel's plans even after a second meeting with him. "I've long since learned not to anticipate anything from Marvin Mandel," Lee said. "You can meet with him for quite a while and not really know what was said."

Mandel's deliberately cautious style was apparent throughout his press conference, amusing many of his longtime observers and friends. Asked whether in retrospect he would have done anything differently during his years in office, Mandel replied, "Not had as many press conferences."

The one point he did make clear was that he would not ask the state to pay his legal fees, although six of his aides have asked the state to pay the legal expenses they incurred while federal prosecutors were investigating Mandel. A special commission has recommended that the state pay the legal fees of two of them.

The celebration began for Mandel shortly after the news came in Thursday and stretched far into the night at one of his favorite retreats as governor, the pentnouse of the Annapolis Hilton Hotel. There, he held down a large table in the far corner, receiving a stedy procession of well-wishers and chatting with reporters about politics and public morality.

This morning, after a sleepless night, Mandel, his wife Jeanne and two of her sons -- 22-year-old Phillip and 13-year-old Paul -- returned to Annapolis for a breakfast at Chick and Ruth's Delly. There, at booth known as the "governor's office," Mandel ate a bagel and made some calls on a red telephone placed on the table by proprietor Chick Levitt.

Almost two hours later, the five "roadrunners" drove in from across the state for coffee and doughnuts at Mandel's Severna Park home. Sitting by a fire in the den, they joked about getting back into the state pension system for the next five days and returning to the State House as the "transition team" that will hand over the government to Gov.-elect Hughes next week.

As they departed for the State House, they resolved to act "with humility" -- a pledge that lasted only as long as it took Silver and Schreiber to reach their car. Recalling the long dismal walk from their offices to an Annapolis restaurant that day of Aug. 23, 1977, when Mandel was convicted, Silver turned to Schreiber and said, "Screw humility. Let's have some fun."