Rep. Barbara A. Mikulski (D-Md.) announced yesterday that she would run for the seat of retiring Republican Sen. Charles McC. Mathias Jr., as she began her second quest to become the first female senator from Maryland.

"I want to be the senator for all the people of Maryland -- the white collar, blue collar, pink collar, new collar and no collar," she said in a congressional committee hearing room. It was packed with activists such as Irene Natividad, president of the National Women's Political Caucus, and former representative Bella Abzug, now a New York lawyer.

Mikulski, 49, said the women were there "in support of sisterhood and solidarity. They were there to see Sally Ride take off. Now they're here to see Barbara Mikulski take off."

This is Mikulski's second bid for the U.S. Senate. In 1974, while a member of the Baltimore City Council, she tried unsuccessfully to unseat Mathias. Mikulski received 44 percent of the vote.

Meanwhile, Maryland House Speaker Benjamin L. Cardin indicated that he is having second thoughts about running for governor. Cardin, who is thought to be a distant third possibility behind Baltimore Mayor William Donald Schaefer and Attorney General Stephen H. Sachs for the Democratic nomination for Maryland governor in 1986, has been under pressure from Democrats to run instead for Mikulski's 3rd District House seat.

"My game plan is to run for governor," Cardin said yesterday. He said he will not change his plans if he concludes he can win. Otherwise, Cardin said, he will consider other options, including filing for Mikulski's seat or remaining in the House of Delegates.

Mikulski, who was first elected to the House in 1976, is the third Democrat to declare in the highly charged Democratic Senate primary. Rep. Michael D. Barnes has announced he will run, giving up what was considered a safe House seat representing Montgomery County and sparking a battle there for his 8th District congressional seat.

Baltimore County Executive Donald Hutchinson also has announced his bid for the Senate, and Gov. Harry Hughes has formed an exploratory committee for the Senate race.

In contrast, Republicans have had problems recruiting prominent candidates for the race, and many Republicans fear that the GOP may lose the Senate seat held by Mathias since 1968. No Republican has announced for the seat.

Mikulski, the granddaughter of Polish immigrants, got her start in politics as a community activist in the Polish section of east Baltimore. Yesterday she made her first announcement from the steps of her Fells Point home, and then headed to Prince George's County, Frederick and Capitol Hill.

Mikulski, who is known as a feisty campaigner, referred to herself yesterday as someone who "cannot be daunted or deterred." She added, "Barbara Mikulski has never been afraid of the big boys."

During her political career, Mikulski has often aligned with groups representing labor, women and consumers. She describes herself as a "common-sense Democrat" who would concentate her energies in the Senate on deficits in the federal budget, trade, research and development, and education.

"I'm proud to be a Democrat," Mikulski said.