Donald B. Robertson, a veteran Montgomery County legislator who over the years has come to symbolize his suburban county's "good government" style of politics, was appointed majority leader of the Maryland House of Delegates yesterday by Speaker Benjamin L. Cardin.

Te selection of Robertson for the leadership post gives Montgomery County more power than it has wielded in the General Assembly for several decades. It also reaffirms Cardin's desire to break from the traditional way of doing things in Annapolis.

Robertson, a 47-year-old lawyer, is a meticulous man who is considered an expert on the rules and procedures of the House. His penchant for following what he believes to be the proper course -- no matter how trivial the matter at hand -- has sometimes frustrated many of his more pragmatic colleagues.

"Don can be exasperating, he can drive you up a tree at times the way he goes over the fine details," said Del. Gerard Devlin (D-Prince George's). "But he's regarded as a decent, good-hearted guy. The fact that he could be majority leader shows that when Ben Cardin became speaker there began an absolutely new era in the House of Delegates."

Traditionally, majority leaders in the House of Delegates have had little use for the "good government" ethic that Robertson espouses. The last majority leader, Del. John S. Arnick of Baltimore County, often made the Common Cause lobby the butt of his jokes.

Cardin, the 35-year-old Baltimore lawyer who was elected speaker last month, said he respected Robertson's "broad background in the rules of the House" and his "ability to handle floor debates."

Although Cardin and Robertson said they had not yet decided precisely how the new majority leader will perform his duties, several House members speculated that Robertson would be used more to steer floor debated than to round up votes in the cloakrooms.

Robertson, however, said that he was not afraid to use the power of his new position. "I was the county delegation leader," he said. "I've had experience at rounding up votes and I'm sure I'll do it some more."

Robertson, the first House majority leader from Montgomery County in several decades, is one of three delegates from the affluent suburban Washington jurisdiction to be appointed to leadership positions by Cardin. Del. Joseph Owens was reappointed chairman of the House Judiciary Committee and Del. Helen Koss was given a leadership slot on the Constitution and Administrative Law Committee.

"This has got to help our county's position in Annapolis," said Del. Ida G. Ruben. "We've ften had problems in the past because the House has treated the county as apart from the rest of the state, as the wealthiest county in the country. I think with Robertson and the others we'll get a better shake."

The other suburban Washington county, Prince George's, also received three leadership positions in the Cardin shakeup. Del. Fred Rummage was named chairman of the Economic Matters Committee, while Dels. Gerard Devlin and John W. Wolfgang were given vie chairmanship of the Ways and Means and Judiciary committees, respectively.