The campaign for Maryland's U.S. Senate seat effectively began today as Gov. Harry Hughes and Rep. Michael D. Barnes each indicated his intention to run for the seat now held by Republican Charles McC. Mathias Jr., who is retiring.

A third Democratic candidate, Baltimore County executive Donald P. Hutchinson, is ready to announce his candidacy at a news conference Tuesday in the county seat in Towson, according to published reports.

Rep. Barbara Mikulski of Baltimore, the fourth Democrat who has been considering the race, made no announcement, but issued a statement saying: "My chances of winning this Senate seat were good even before Senator Mathias withdrew. This race looks even better for me now, as the field of candidates increases. I think it's great for the voters to have a choice."

Hughes, widely considered the early favorite nearing the end of two terms of statewide office, stopped short of formally announcing his plans and declined to meet with reporters.

Nevertheless, he issued a statement saying that he had been urged by friends and supporters to seek the seat and "in recent weeks the voices advocating this step have grown stronger and more numerous."

Hughes's statement said he has authorized creation of a "Harry Hughes for Senate" committee under the chairmanship of Fred L. Wineland, former secretary of state and the treasurer of his state campaign committee.

The committee will raise funds and consolidate political support "in advance of formal, official declaration of candidacy next year," the statement said.

The committee cochairmen listed include former Defense secretary Clark Clifford; former U.S. attorney general Benjamin R. Civiletti; state Sen. Clarence W. Blount (D-Baltimore), majority leader of the state Senate; and Mathias J. DeVito, chairman of Rouse Co., developer of Baltimore's Harborplace.

Barnes, a liberal Montgomery County lawyer who has achieved a national reputation for his foreign policy expertise, will formally announce his candidacy next Monday with stops on Capitol Hill and around the state, his spokesman Bill Bronrott said today.

Several supporters said Barnes told them of his decision at a meeting Saturday, hours afer Democrats at a party conference in Anne Arundel County gave him a standing ovation for an extemporaneous speech on the importance of sending a Democrat to the Senate.

Barnes told reporters and party activists that day that he was "strongly leaning" toward entering the race.

"Mike is very excited, we all are," said Montgomery County activist Stan Gildenhorn, a former chairman of the county's Democratic Party.

"We consider it a challenge but a winnable challenge," Gildenhorn said.

None of Hutchinson's staff would confirm reports that Hutchinson plans to seek the Senate seat.

Nevertheless Hutchinson is the only one of the four potential candidates who has notified the secretary of the Senate of the creation of an exploratory committee to raise funds for a Senate race.

One other person has filed a statement of candidacy, Edward M. Olszewski, who could not be reached for comment.

Though Hutchinson lacks both the statewide name recognition of Hughes and the national stature Barnes brings to the race, he potentially controls a large block of Democratic votes and a commanding political organization as leader of one of the state's largest counties.

Hutchinson is barred from succeeding himself as county executive.

Over the last several months, he has been considering a future that most political observers assumed was directed toward winning a slot as lieutenant governor on a ticket headed by Baltimore Mayor William Donald Schaefer or a congressional race against first-term incumbent Helen Bentley, a Republican.

Hutchinson told a reporter at Schaefer's million-dollar fund-raiser last week that he would prefer to be governor but, with Schaefer in the race, he could not win.

With that option foreclosed, he said, "The conventional wisdom is that I'm going to run against Helen Bentley. Everyone has accepted the conventional wisdom except me."

Instead, he said, he would "do the same type of thing Mike Barnes is doing, traveling around, begin to assess the senatorial race."

Several Democrats at Saturday's party conference said Hughes has left open the possibility of changing his mind about a Senate race.

Though early polls show Hughes with a clear lead over other Democratic rivals, some believe his involvement with the lingering savings and loan crisis has weakened his support.

Though Democrats face a degree of uncertainty about their candidate over the next few months, Maryland Republicans face an even more unsettled situation with the loss of two members of their congressional delegation, Mathias and Rep. Marjorie Holt, who is also retiring.

GOP leaders are hoping to persuade former U.N. ambassador Jeane Kirkpatrick to run for the seat.

Kirkpatrick was out of town and unavailable for comment.

"I think Republicans around the state are waiting for her to do it," said state party chairman Allan Levey.

"I think she'd be at least an even chance, if not the favorite, to win against any Democrat . . . . She's conservative on economic and defense issues and moderate on social issues. She'd fit right in," Levey said.