Representatives of 15 Baltimore area labor groups held a news conference Monday to back Rep. Barbara Mikulski in her bid for the Democratic nomination for the U.S. Senate. Union officials said they would work to get the endorsement of the Maryland AFL-CIO, whose members are scheduled to vote to endorse one of the Senate candidates at its convention in July.

Among those participating in the news conference were locals representing the bricklayers, bakery workers, communications workers, electrical workers, hospital workers, seafarers, steelworkers, teachers, transit workers, and united food and commercial workers. Townsend Releases Tax Return

Kathleen Kennedy Townsend, a candidate for Maryland's 2nd District congressional seat, released the joint federal tax return she and her husband, David, filed this year, revealing they made a total of $174,434 in 1985.

Most of the Townsends' income came from trusts and partnerships, on which they paid $45,594 in state and local income taxes.

The Townsends, who have three daughters, claimed deductions of $72,297, including $27,012 in mortgage interest and $22,012 in contributions to charity, including $19,810 worth of realty corporation stock to the Joseph P. Kennedy Jr. Foundation.

Townsend, the eldest daughter of the late Robert F. Kennedy, is challenging U.S. Rep. Helen Delich Bentley, the Republican incumbent, and Linda Bach Robinson in the congressional race. Townsend called on her fellow candidates to make their tax returns public. Del. Parlett Stepping Down

State Del. John K. Parlett, who has represented St. Mary's and Charles counties since 1981, said last week he will not seek reelection this fall.

"Pressing personal and family obligations will not allow me to continue to devote the time and effort to the legislature that the people of the 28th District have the right to expect from their elected representatives," he said. Announcement

Democratic Del. Marilyn Goldwater of Bethesda has announced she will run for the state Senate from the 16th District in lower Montgomery County. Currently the deputy majority whip, she has been a delegate for 12 years and is a registered nurse. She serves on the Environmental Matters and Joint Federal Relations committees.