The prosecutor who helped convict former Vice President Spiro T. Agnew and former Maryland Gov. Marvin Mandel finally said today what has been on his mind for the five years he spent chasing down corrupt politicians.

Maryland citizens are to blame for the "festering" corruption in their state. Assistant U.S. Attorney Ronald S. Liebman said, and so are the local prosecutors who are "inexecusable" for failing to pursue corruption.

In a speech at the governor's Club here, Liebman said that Maryland's political boss system, financed by questionable campaign contributions was at the heart of the state's problems.

"There is a fine line between a campaign check and bribery. Over the passage of time the line becomes hazy and soon it disappears," he said.

Liebman is leaving office shortly to go into private practice and he chose a luncheon hosted by the Maryland Conference of Social Concern for his farewell.

It was a speech about his own naivete. On his 30th birthday, four years ago, Liebman was at a Justice Departmaent press conference the day after Agnew resigned. "I thought, with this kind of national media attention, the public humiliation, it's going to be a long time in Maryland before someone else commits this kind of crime."

Instead, he found that "corruption had become such a way of life . . . that the participants were actually crass about it."

In Liebman's view, citizens continue to give votes to local bosses who sell them in a package to politicians who in turn get their money from corrupt businessmen.

"It makes corruption a very very expensive commodity," he said. Campaigns cost a lot of money and only businessmen can afford them, businessmen who care more about favors than what a politician stands for, he added. "Most don't stand for anything, anyway."

His own investigations with assistant U.S. Attorneys, Barnet D. Skolnik, Daniel J. Hurson and earlier, Russell T. Baker Jr., uncovered the sore of corruption, Liebman said, and once they dug into it "the sore burst."

The big question at cocktail parties . . . is more or less corrupt than any other state. What difference does it make if Maryland is more corrupt than New York or Montana?" he asked.

Liebman thinks the proper question is how to clean up the state and keep it clean. He came up with a host of suggestioned solutions: public financing of campaigns with drastic reductions in cost of running for office, appointment of local, prosecutors so they aren't required to campaign for office, stiffer jail sentences and higher salaries for elected officials.

He returned repeatedly to what he described as the failures of local prosecutors. "To put it mildly, they haven't made this kind of prosecution a high priority item." he said. "They are too closely aligned with the political system now."

Federal and occasionally state prosecutors have been responsible for almost all the corruption convictions, despite the fact that the crimes involved have been essenially local are generally part of the dominant political machinery in their countries, have argued that they are not equipment to pursue complicated white-collar cases but that they would if they could.

Liebman refused to single out any county or city in Maryland to illustrate his point. He said only that some state's attorneys complain that they don't have the resources to go after corruption.