IS MARYLAND POLITICS irretrievably corrupt? A lot of people are bound to be wondering in the wake of this week's conviction of Gov. Marvin Mandel and his friends. And they will find little encouragement in the long list of Maryland officials - now including two governors, a senator, a congressman, several state legislators and two county executives - who have either been convicted of crimes involving public business, or pled guility or been charged with such crimes and have offered no defense. It is hard to avoid suspicions that the Free State is perpetually for sale or rent.

Why Maryland? Political scientists and historians might point to several factors that can easily spur corruption. The state is fairly small. Its politics has been dominated by one party and largely managed through clubs - both country clubs and the ward clubhouses of Baltimore. Its rapid growth has brought a vast expansio of state projects and services, promising large profits for those with access and influence in Annapolis. Yet its legislators are still part-timers and its governor is grossly underpaid. This volatile combination of factors has produced government that, while often competent, is married by ethical obtuseness and a failure to comprehend what is so wrong about cronyism and the mingling of public and private affairs. That is what brought down Marvin Mandel - and that is what the governor as well as some of his longtime friends still do not seem to understand.

Government by favor and friendship is hardly unique to Maryland. Indeed, residents of several other states might say Maryland is distinctive only in that more of its corrupt politicians have gotten caught. That does point to one of the state's great countervailing assets: the strength of the federal prosecutor's office in Baltimore and the determination with which several U.S. attorneys and their staffs have pursued official corruption over the past 15 years. The only sorry aspect of that record is that the job has fallen to federal prosecutors by default. With few exceptions, state prosecutors have shown virtually no interest in what are, after all, also crimes against the state.

Other signs of progress exist, but are also qualified. This summer, for instance, the state finally managed to pick a management contractor for the huge Baltimore subway project on the basis of merit. But that came about only after contractor Victor Frenkil and the Board of Public Works had tried to manipulate the selection process so flagrantly that Transportation Secretary Harry Hughes resigned in protest.

Similarly, it may be considered a gain that State Sen. Roy N. Staten, a longtime Mandel ally, has just decided not to seek the state-cabinet-level post that Mr. Mandel had promised him. That, however, leaves Mr. Staten in charge of the state Democratic Party machinery, which suggests that campaign business may go on as usual next year.

Acting Gov. Blair Lee III does seem to recognize the political desirability of detaching himself from the squalors of the past. But mere pronouncements about no-more-corruption will not be enough; the real question is whether Mr. Lee, is his old-shoe style, can instill and sustain a new spirit in Annapolis. The answer may not be evident until next winter when the legislature meets.

Beyond that is the larger question of what new leadership may emerge now or in next year's campaigns. The younger generation of Democrats, especially in the Washington and Baltimore suburbs, includes several ambitious people who seem to have different standards as well as different styles. The Republicans claim to see oportunity, though they are still looking for a strong gubernatorial candidate. In terms of changing the climate, though, the greatest potential influence could be the prominent public officials in both parties who are recognized as people of integrity - and who have largely avoided getting involved in the grubbier aspects of state government. It is about time for them to make clear that the old ways of running Maryland simply are not acceptable any more. Ultimately, of course, we would hope that the same message will come from the voters of the state, for they are the real victims of the cronyism and corruption that have persisted so long.