Acting Gov. Blair Lee III, who will become Maryland's full-fledged chief executive when Marvin Mandel resigns or is sentenced, moved quickly yesterday to separate himself from the convicted governor.

Moral leadership, said Lee at an afternoon press conference in response to a reporter's question, "was probably never one of (Mandel's) strong points."

Mandel's conviction yesterday for racketeering and mail fraud reshaped the political landscape of Maryland at the approach of a new election year. It shored up the position of Lee, whose gubernatorial campaign billboards already dot the state, and provided Republicans with new hope for winning the governor's seat next year.

And the jury's verdict against some of Mandel's closest friends and political associates removed from Maryland politics its biggest fund raisers, potentially helpful for candidates with fewer financial resources.

While Mandel's allies yesterday reasserted their belief in the governor's innocence and predicted his vindication on appeal. Democratic aspirants for his seat - except Lee - generally sounded more cautious and neutral.

In press conferences and interviews, they carefully avoided criticism of both the jury verdict and Mandel.

Republicans spoke in somber tones about the corruption of a Democratic adminstration, said that Lee cannot escape identification with his convicted former running mate and predicted that Lee's clear incumbency as chief executive will narrow the crowded field of Democratic contenders for governor.

David Forward, the Republican state chairman, termed the jury verdict "correct" and said, "We have to do something to change the entire structure of the way the state is run . . . Only a new Republican state administration can change the whole structure . . . I think the Republican Party is the reform party."

At this time, the only announced Republican contender for governor is John Harwick, a Harford Country attorney who formerly served in the House of Delegates. Others Republicans whose names have been mentioned as possible candidates include Anne Arundel County Executive Robert Pascal, Montgomery County Executive James P. Gleason, former U.S. Sen. J. Glenn Beall and his younger brother George Beall, the former U.S. attorney who initiated the Mandel probe.

Both Republicans and Democrats acknowledged that the state's image is badly tarnished by corruption involving figures in both political parties in recent years.And persons in both parties expressed compassion for the men convicted yesterday.

Lee called it "a sad day for all Marylanders," Senate Minority Leader Edward J. Mason (R-Western Maryland) observed, "All of us are glad this ordeal is over." And Ted Venetoulis, the Democratic county executive of Baltimore County who is seeking the governorship, described himself as "drained, I just feel empty inside . . . It is a tough human thing the drama of it all."

Democratic leaders, including Lee, turned away questions about the political implications of the verdict as untimely and inappropriate.

"There is no doubt it has tremendous impact on the government, the citizenry, the political structure," said Attorney General Francis B. Burch, Mandel's running mate in 1970 and 1974 and now a gubernatorial aspirant, "but ut is highly inappropriate to discuss now what the impact would be on my plans or anybody else's plans."

State Senate President Steny Hoyer (D-Prince George's) also refused to discuss what impact the conviction would have on his campaign for governor.

But Hoyer, Burch and other Democratic leaders pledged their cooperation and support to Lee during the immediate period ahead. In his own race to retain the office next year, Lee will have a full legislative session under his belt. That is considered a political advantage that could outweigh the time it will take away from his campaigning.

At his press conference yesterday, Lee appeared very much the chief executive, promising that "the job of carrying on state government will go on without interruption or consternation or confusion . . ."

Lee, appearing in the governor's reception room at the State House, said he phomed Mandel at 1 p.m. after the verdict and the two had a "brief conversation." They will meet again today, Lee said, "with the view of discussing the problems in the transfer of powers such as it is."

Since Mandel in effect transferred all powers to Lee June 4, the acting governor said, "There will be no ripples in the water . . ."

Later in the press conference, however, Lee said in response to a question that he is considering asking Mandel's staff to resign. "I have some ideas along the lines of sanitizing government, but you're way ahead of the game," he said.

Lee, while declining comment on how "this tragic occurrence" would affect next year's campaign, allowed himself to be drawn into a discussion of Mandel's moral leadership in relation to the state's tainted image.

"This is a relatively brief history" of corruption "when you think of the long history of this state," said Lee, whose patrician lineage includes both the Blair and the Lee families and is deeply rooted in Maryland. "My No. 1 priority is going to be to regain the old image, the old good name of the state of Maryland which existed for centuries before this all started."

Restoring confidence in the integrity of Maryland's government is "mainly . . . a matter of leadership and example," he said. "That situation point, "was probably never one of (Mandel's) superstrong points. He just never believed in candor, and without candor you have problems. Being a good governor isn't quite enough.

"You have to be scrupulously honest . . . to avoid the appearance of any corruption and the suspicion of any corruption," Lee said. "The things (Mandel) did . . . I suppose a great many other guys in other states have done the same thing, and that they were simply being nice to their friends. I think you have to be very careful about it."

Lee, a man of independent means who earned $44,800 as lieutenant governor, will take a pay cut to assume the higher post. The job of governor pays only $25,000, a relatively low salary cited repeatedly by Mandel supporters as a defense for his acceptance of gifts from political associates.

Judge Allen B. Spector of the Baltimore City District Court, a close personal friend and longtime political assosciate of Mandel, said the governor was a victim of new political standards.

"It's like they changed the rules in the middle of the ball game," Spector said yesterday. "In politics, we were all raised with the idea that the guys who helped you, you favored."

"If doing favors is going to be an indictable offense in the future," said State Sen. Harry McGuirk, a South Baltimore ally of Mandel's, "then I think every elected official is going to have to reconsider what he does officially and unofficially."

Remarked House Speaker John Hanson Briscoe (D-St. Mary's): "The lesson is while these things might seem to be casual, there are people looking at you."

Moral leadership, Lee said at this is (that) being a good governor is great but not good enough . . . "

Senate Minority Leader Mason was asked why 11 high Maryland politicians have succumbed to political corruption investigations in little over a decade. "We've either got excellent prosceutors or an awful lot of corruption in high places," he said. "It looks like it's proven to be both."