Rep. Michael Barnes (D-Md.) yesterday launched his race for the seat of retiring Republican Sen. Charles McC. Mathias Jr. in a series of speeches across the state, vowing to run "an all-out, aggressive, vigorous" campaign.

Meanwhile, in Baltimore, at an annual AFL-CIO convention, Rep. Barbara Mikulski (D-Md.) said she "would be announcing my intention in a few days, and I think you can guess pretty much what it is.

"I'm a fighter. I've never been afraid to take risks," Mikulski said. "Politics is not a spectator sport and Barbara Mikulski has never been one to sit on the sidelines."

Barnes, 42, is the second candidate to formally enter the race for the Democratic nomination. Baltimore County Executive Donald Hutchinson last week announced his bid. Gov. Harry Hughes has formed an exploratory committee.

No Republicans have announced yet for the Senate race. Maryland Republicans had hoped that former United Nations ambassador Jeane Kirkpatrick would run for the seat, but last week she took herself out of the running.

Barnes is enormously popular in Montgomery County, where he has won reelection with 71 percent of the vote in the last two elections. And nationally he has emerged as a leading spokesman for his party on Central American issues.

But he has played a smaller role in statewide politics, and his task now is to win support in other parts of Maryland.

Mikulski, 49, a former social worker and Baltimore City Council member with a reputation as tart-tongued, witty and occasionally abrasive, was elected to Congress in 1976, two years after she ran unsuccessfully for the U.S. Senate as the Democratic nominee.

She has long been active in national party affairs and is a strong advocate of labor and women's rights, while maintaining a reputation for thorough constituent service.

Mikulski has been traveling the state for a year trying to drum up support for her Senate candidacy. Both she and Hughes have previously run statewide races, which Barnes has not.

Barnes, however, appeared unperturbed yesterday by questions of whether his low-keyed Montgomery County style of campaigning will sell in Baltimore or the Eastern Shore or in western Maryland.

"Three weeks ago, no one had heard of Gloria, but now everyone has," Barnes said yesterday morning at a Capitol Hill news conference. "Everyone will know by next September who Michael Barnes is. I'm going to be in Baltimore City so much that everyone will think that I live there."

Barnes refused to predict name recognition and popularity a year before the primary. He noted he had only 2 percent name recognition in 1977 when, as a member of the Maryland Public Service Commission, he beat GOP incumbent Rep. Newton I. Steers Jr. "even though almost everyone agreed I could not win."

Yesterday's sweep through Baltimore, Salisbury and Annapolis was intended as the first step in Barnes' effort to make himself known beyond the Montgomery County borders. He plans to appear in Hagerstown early today.

At the AFL-CIO rally in Baltimore, Barnes was praised for his strong labor record. Mikulski, who blasted the Reagan administration's trade policies, also was warmly received by the labor group.

Barnes said yesterday that he had been thinking of running for the Senate for about a month but that his "interest would have been substantially diminished" if Mathias had run for reelection. He took pains not to criticize Mathias, though they belong to different parties. "We've had a history of excellent representation in the Senate," he said.

Barnes estimated that the primary race would cost $1 million to $1.5 million. He said he has raised a "substantial amount of money already," but declined to say how much.

Barnes' decision not to seek House reelection has ignited a scramble for the 8th District congressional seat in Montgomery County. Democrats mentioned as potential successors include state Sen. Stewart Bainum Jr., a wealthy motel and nursing home executive; Rockville lawyer Victor Crawford, who served in the state Senate for 16 years; Democratic National Committeeman Lanny Davis, a lawyer who ran in 1976; Wendell Holloway, a Ford Motor Co. executive; and Leon G. Billings, a former Senate subcommittee staff director.

Republican state Del. Constance A. Morella, of Bethesda, has said she will declare her candidacy.