Maryland Republicans at the party's state convention here this weekend don't know whom they will nominate for governor next year, but they appear to be in agreement about whom they are going to run against - outgoing Governor Marvin Mandel.

From the podium at the convention center to the informal sessions around the pool at the Sheraton-Fontainebleau Hotel here, the GOP's central committee members were rubbing their hands in gleeful anticipation of running as the party of reform in corruption-ridden Maryland.

"It's no easy trick, some admit, because the Republicans like Lady Macbeth, must rub out the imprinted "dammed spot" that lingers from Mandel's predecessor, Spiro T. Agnew.

The most outspoken proponent of an anti-Mandel, anticorruption campaign was he convention's keynote speaker, Donald J. Devine, the state party's parliamentarian.

"Governor Mandel is the best asset we have," said Devine, a University of Maryland political science professor from Montgomery County. "We're crazy if we don't take advantage" of Mandel's indictment and trial on political corruption charges, Devine said, "Even if he is not convicted, everyone knows there's something behind it," Devine said.

There's been enough corruption in Maryland," Devine said.

Devine conceded that "unfortunately, we're not totally blameless here," and apparent reference to former governor and Vice President Agnew and former Anne Arundel County executive Joseph Alton, a Republican who recently served a prison term for accepting kickbacks.

There was light shed on whom the Republicans might nominate to carry the attack to the Democrats. One avowed and several others who wouldn't deny their interest, booked the hospitaltiy rooms at the hotel here last night.

John W. Hardwicke, former Harford County councilman and delegate to the General Assembly, invited all of the 130 convention-goers to a party during which he said that although he has not gone through the formality of declaring his candidacy, "the reality of that I am a candidate - period."

Hardwicke predicted that it will not be necessary for the GOP nominee for governor to make "honesty and overt issue because the people will run the Republican against Marvin Mandel."

Hardwicke, a white-haired corporation lawyer who said he represents his clients in energy matters, apparently will follow the technique that lifted Louise Gore to the Gubernatorial nomination in 1974.

Miss Gore, looking doznes of pounds slimmer than when she ran against Mandel three years ago, merely smiled politely when asked if she might make a second try. Among the delegates here, especially those from her home in Montgomery County, she apperared to have a residue of support.

Several of Hardwicke's supporters worry privately that he may repeat what some party regulars believes was Miss Gore's major error, of concentrating on gaining support of the party's leadership but never broadening the appeal to capture the rank and file vote. The technique may be good enough to get the nomination, but in Maryland, where the Democrast have a substantial registration edge, it is not enough to get elected.

The man who was the victim of Miss Gore's primary victory, former Prince Georges County Congressman Larry Hogan, also was here this weekend. While Hogan said his apperance was as the party's national committee-man he was being touted as the condidate by some member of the Prince George's Central Committee.

Former United States Senator J. Glenn Beall couldn't dismiss his appearance here, as Hogan and Miss Gore could, as part of an official duty. Beall, sporting a tan that results from at least twodays a week on the golf course in his home town of Frostburg, where he has returned to his family's insurance business, said merely that iwht the primaries still 15 months off, there is "plenty of time" for a candidate with name recognition to declare.

A handful of persons who were not here also were being mentioned as possible candidates: Anne Arundel County Executive Robert Pascal, Montgomery County Executive James Gleason, former U.S. attorney George Beall, the former senator's younger brother, and even U.S. Sen. Charles Mathisa.

Although Mathias said earlier this week that he wasn't interested in the governorship - and former Senator Beall said it would be rare for a senator to move from the Capitol to the State House - Baltimore County Republican Chairman Malcolm M. McKnight was passing out black and yellow "Mathias for Governor" bumper stickers.

A Mathias aide who attended the convention said McKnight is a good friend of the senator who was acting with Mathias' knowledge but without his approval or encouragement.

State Sen. Edward Thomas of Frederick, one of three Republican legilators here (the others were Sen. Howard Denis of Montgomery and Del. George Price of Baltimore and Del. George Price of Baltimore County) said Mandel's programs, such as the large increase in state employees and the expensive and controversial Baltimore subway, rather than Mandel's personality, would be the issue next year.

But however they phrased it, the Republicans here were savoring the prospect of using Mandel as the target of their 1978 campaign.

"Only a Republican can break the system of corruption," keynoter Devine said. "All of the Democratic candidates are connected with the Mandel administration."