Maryland Sen. Paul S. Sarbanes and Maryland Rep. Barbara A. Mikulski have decided to support former vice president Walter F. Mondale in Mondale's drive to become the next U.S. president, and will campaign for him nationally.

Mondale, who will be campaigning in Maryland today and appearing at a fund-raiser tonight in a Baltimore suburb, officially will name Sarbanes as chairman of his Maryland campaign, according to Mondale's and Sarbanes' aides.

Mikulski and Maryland Rep. Michael D. Barnes will co-chair the state drive. Barnes, who represents Montgomery County, and Maryland Attorney General Stephen H. Sachs, endorsed Mondale's candidacy weeks ago and will be on hand for his fund-raiser tonight.

The former U.S. senator and vice president under Jimmy Carter already is considered the front-runner among half a dozen Democrats seeking the party's nomination.

The early endorsements he has pinned down from leading liberal Democrats in Maryland may orchestrate a crucial "bandwagon" effect for his campaign here and in other states, according to his supporters.

"I think it's imperative that Democrats win the presidency in 1984, and it's my perception that I can make a contribution by joining the campaign now and doing what I can to help," Sarbanes said yesterday. "Good people have to help other good people."

Sarbanes, spicing up his praise for Mondale with attacks on the Reagan administration, said he concurs with the assessment that Mondale made a month ago in Minnesota, where he formally announced his presidential candidacy. Sarbanes added: "I think he is ready to be president, and I think the country is ready to have him be president."

Mikulski, a longtime supporter of Sen. Edward M. Kennedy (D-Mass.), who took himself out of the presidential contest, said that she will endorse Mondale today "enthusiastically, whole-heartedly . . . . I've always liked him; he's my kind of president."

The Baltimore congresswoman said that support from Sarbanes, herself and others among the state Democratic Party's more liberal officials cannot help but boost Mondale.

"I don't think my constituents automatically go with what I say," Mikulski explained, "but they're going to think: 'There must be something to this guy if she's for him.' "

In addition to Mikulski and Barnes, additional state campaign cochairmen are expected to be announced in the future, a Mondale aide said yesterday.

But Maryland Gov. Harry Hughes will not be endorsing Mondale at today's campaign gathering, an aide to the governor said.

"He likes him, he's going to be meeting with him," was all that Lou Panos, Hughes's press secretary, would say.

Another presidential aspirant, Sen. Alan Cranston (D-Calif.), has asked to meet with the governor, according to a Hughes associate, "and Hughes doesn't want to be discourteous."

Mondale supporters said they are "hopeful" that Hughes, who has talked about Mondale as having the only statewide Democratic presidential organization, soon will join the Mondale campaign.

The former vice president will be in Annapolis and Baltimore today and tonight and, in addition to meeting with Hughes, will meet with leaders of the General Assembly and other legislators.

State Sen. Laurence Levitan (D-Montgomery), a Mondale backer, said that five or six of his colleagues support Mondale's candidacy but that "many others have just not made up their minds."

But Nathan Landow, a wealthy real estate developer from Montgomery County who has been helping to raise money for Mondale's presidential bid, said that endorsements by Sarbanes and Mikulski should kick off "a great day" in Maryland.

Landow said that from 800 to 1,000 supporters are expected at today's fund-raiser.