When Democratic vice-presidential candidate Geraldine A. Ferraro appeared at a downtown political rally here last month, she recalled that one of her slogans when she first ran for her U.S. House seat was "Send a tough Democrat to Congress."

"And then I got to Washington," Ferraro continued, "and met Barbara Mikulski."

Even allowing for election-year hyperbole, Ferraro's assessment of the congresswoman from Maryland's 3d District had the ring of truth to it. In her first political outing, the diminutive Mikulski parlayed her community activism and her successful opposition to a proposed highway in her East Baltimore neighborhood into a seat on the City Council. Today, after four terms in Congress, she is still as scrappy and combative as they come.

But now, 13 years after winning her first political race, the former social worker also has become a recognizable national figure: a leader in the women's movement, a close friend of Ferraro, an adviser and key campaign aide to Democratic presidential candidate Walter F. Mondale, and a party spokeswoman prominent enough to address the Democratic National Convention three times in four days.

Approaching what many political observers predict will be almost certain reelection to a fifth term on Nov. 6, she is increasingly being viewed as a likely candidate for the U.S. Senate in 1986 when Republican Sen. Charles McC. Mathias Jr. comes up for reelection.

None of this appears to daunt Ross Z. Pierpont, Mikulski's Republican opponent and a hardy perennial of Maryland politics who has been regularly sprouting -- but failing to bloom -- for almost two decades. Under two different party labels (he was a Democrat until 1970), Pierpont has lost races for mayor of Baltimore, for Congress, for U.S. Senate and for governor.

"This time I expect to win," said the self-made millionaire businessman and surgeon who asserts that he has spent $800,000 running for office over the years. It is a familiar refrain. "My name identification equals former governor Marvin Mandel's," Pierpont declared in 1974, shortly before he was trounced by Mathias in the Republican Senate primary.

Despite a 6-to-1 Democratic voter registration advantage in the district, which sweeps across the ethnic strongholds of East Baltimore, the predominantly Jewish areas in northwest Baltimore and the suburban enclave of Columbia, Pierpont believes that Mikulski's "liberal socialist" politics are anathema to her constituency.

"The people are Democrats, but they are Democrats the way I used to be," said the 67-year-old Pierpont. "They are conservative, church-going, family people. . . . They are not oriented to big spending and giveaways."

A blunt, outspoken conservative who supports President Reagan on most issues, Pierpont dismisses his opponent as "wind, bombast and nothing; she has no record of accomplishment."

Mikulski defends her politics -- which combines classic New Deal economic and social welfare beliefs with strong opposition to many of Reagan's foreign and military policies -- by noting that she has averaged 75 percent of the vote in her general election contests for the House. "If three out of four people vote yes for Barbara Mikulski, they must feel pretty satisfied," she said.

In the 3d Congressional District, voters apparently have been satisfied with Mikulski's brand of politics since they first elected her predecessor and ideological twin, Paul Sarbanes, now a U.S. senator, to the seat in 1970. But where Sarbanes' style is studied and professorial, Mikulski's reflects her street activist roots: feisty and direct.

"Her style may put people off," said one longtime Democratic activist, "but in the Baltimore area people kind of like intensely personal politics. She made her mark as an advocate, and the voters see her in that vein."

"I'm sure her character and personality aren't universally beloved," added Robert C. Embry Jr., a longtime Mikulski ally who served on the Baltimore City Council and later as the city's housing director. "But nobody's are. She and Sarbanes have very different personalities, but both are very honest and intelligent and open-minded. Those are more important characteristics than if they are fiery or contemplative."

If Pierpont shares anything with Mikulski, it is a constitutional inability to mince words. Some unvarnished Pierpont-isms:

"Mikulski doesn't give a damn about this district. Everything she does is for Barbara Mikulski."

"The reason she was elected before is that she was cute, loud and Polish. But she's none of those things except Polish and loud."

"She is promoting herself for the Senate. She has no more right to be in the United States Senate than a stray dog."

"The Equal Rights Amendment is an operation whose time has long since passed. . . . I don't think they ERA proponents have made a case for anything but the female aberrationals. People say Barbara Mikulski represents women, but she doesn't represent any women I know."

Mikulski, who weathered a far more vicious primary challenge two years ago by a follower of right-wing ideologue and sometime presidential candidate Lyndon LaRouche, seems unperturbed by her opponent's charges. Ticking off some of the development projects that have transformed Baltimore in the past decade, Mikulski touts her partnership with Mayor William D. Schaefer and other elected officials to prove her effectiveness. "I'm not a lone ranger," she says. "I have a record of 13 years of public service."

Mikulski insists she is taking her reelection campaign "very seriously," and said she will match the roughly $175,000 Pierpont expects to spend ($35,000 of it his own).

But many political pros in Maryland feel that Mikulski can well afford to approach her reelection campaign with the kind of equanimity that allowed her to take three days this past week to campaign for Mondale on the West Coast. "She'll win 4-to-1," said state Del. Anthony M. DiPietro Jr., an East Baltimore Democrat.

Even Allan C. Levey, chairman of the state GOP, calls Pierpont a "long shot." Levey believes that Pierpont will benefit from Mikulski's identification "with the ultra-liberal wing of the Democratic party" and President Reagan's coattails, "but whether that means victory or not, we'll have to see."

Peter Marudas, a political operative for Sarbanes, said simply that Mikulski "will have the seat for life."

But many Maryland Democrats assume that Mikulski does not want to grow old in the House, and that in 1986, when she turns 50, she will seek a rematch with Mathias, who overwhelmed her a decade ago when she attempted to move from the City Council to a seat in the U.S. Senate.

Mikulski herself says she has made "no decisions. My plans right now are to get myself reelected and work for the election of Mondale and Ferraro. Then I will very carefully analyze where I can do the most good for the most people."

She also will have to analyze whether she can fund both a Senate primary, possibly against Gov. Harry Hughes, and a six-week general election campaign against the well-entrenched and popular Mathias.

"That would be the major question," Embry said.

But Embry and others believe that Mikulski's role as a co-chairman of the Mondale-Ferraro campaign, combined with her national constituency in the women's movement, could provide her with the contacts and prominence she needs.

Mikulski says her only immediate goal is to renew her "contract with the people" in this campaign. "I don't have a linear career plan," she added.

But it is worth remembering that in 1974 when she first challenged Mathias, one of her slogans was "Barbara Mikulski is not afraid of the big boys."