Baltimore County Executive Donald P. Hutchinson, a two-term moderate Democrat who has earned high marks for his administrative competence, today became the first person, Democrat or Republican, to formally announce his candidacy for the U.S. Senate seat being vacated by Sen. Charles McC. Mathias (R-Md.).

With a plain, direct approach that some call his trademark, Hutchinson opened his remarks to about 200 supporters by saying, "I think each of you knows that I am here to announce that I am running for the United States Senate." He received an instant ovation that was followed by a brief, rousing speech stressing his strong local roots and ability to forge coalitions.

He went on to emphasize his record as a coalition builder and problem solver for the county, and pledged that his campaign will show voters from the rest of the state "what it means to have local roots, a local background, a local education; to have an understanding of what the state really is."

Later in his remarks, the 39-year-old former state senator explicitly acknowledged that one of his first tasks in the race will be broadening his campaign beyond Baltimore County, and to a lesser extent, Baltimore City, where he has worked diligently to develop a strong relationship with Mayor William Donald Schaefer.

That point was underscored by the composition of his audience today, virtually all county lawmakers and activists, with Harford Del. Eileen Rehrman and Harford County Executive Habern W. Freeman Jr. as two exceptions.

Nevertheless, Hutchinson, seeking to emphasize his strengths, maintained that his base in the sprawling, suburban county potentially brings with it command of a huge bloc of 250,000 Democratic votes and a local organization that easily returned him and the entire County Council to office in 1982.

"No one starts with a better home base and an ability to broaden it than I do," he told reporters today. Ideologically, he said, "I think we have something to offer this state that's different from what the other candidates have. I think I bring different background to national issues."

His supporters, predictably, agreed, though some said the difficulties of running the statewide campaign cannot be overlooked.

Prince George's County Executive Parris Glendening, a Hutchinson fan who emphasizes he has not endorsed anyone for the Senate nomination, said, "He is assessed by almost everyone as having done an extraordinary job. I'm not sure loved is ever the right word for the public's attitude to a public official, but he probably comes close to that. But I don't know how important any of that will be when it comes to this kind of race."

By and large, observers agree, Hutchinson has been a very popular executive who has avoided controversy and enthusiastically taken charge of the range of management problems that beset county government. He wins high praise from the business community for his efforts to target capital improvements to high growth areas and to follow through on planned development strategies.

"I think he's been a good county executive," said council member John W. O'Rourke, a three-term incumbent. "It was under the Hutchinson administration that the government carried out a strategy to manage growth."

O'Rourke said Hutchinson has aggressively sought federal funds for urban renewal, reversing an almost isolationist stance of some predecessors. About the only sustained recent criticism of Hutchinson has come over his efforts over the last two years to raise the tax rate. Critics contended he was trying to stockpile a surplus so he could offer a tax cut in his last year in office, and the council refused to go along.