Rep. Barbara A. Mikulski (D-Md.) came in for her harshest grilling yet last week by fellow candidates in the Democratic senatorial primary.

During a radio debate sponsored by the League of Women Voters and Baltimore's WBAL radio station, seven of the eight Democratic candidates were able to question each other for the first time. Mikulski took questions from four of her opponents, including Gov. Harry Hughes and Rep. Michael D. Barnes, her chief rivals in the Sept. 9 primary.

Hughes questioned Mikulski about her votes while a member of the Baltimore City Council to block dredging of the Baltimore Harbor, which she subsequently supported.

Mikulski -- who remained cool throughout the questioning and once jokingly thanked her opponents for turning the forum into the "Barbara Mikulski Show" -- replied that she had had concerns about the environmental impact of dredging.

But she told Hughes that those questions were subsequently addressed "because of your leadership."

Barnes asked Mikulski why a conservative right-wing group had spent "tens of thousands" of dollars on television and newspaper ads earlier this year to discredit his liberal voting record -- and not hers, as the front-runner. Mikulski shrugged and said, "I really can't answer that."

Among the more substantative differences that emerged among the candidates during the two-hour debate, which also featured questions from a panel and listeners, were in views of the death penalty.

Mikulski asked the other candidates to state their opinions, noting that she supports the death penalty for crimes involving espionage, even in peacetime. Both Barnes and Hughes said they oppose the penalty, as did A. Robert Kaufman, a longtime social activist who said he was disappointed in Mikulski's stance.

The other candidates -- Leonard Trout, a retired Social Security Administration worker; Edward Olszewski, who is also retired, and Debra H. Freeman, a follower of political extremist Lyndon H. LaRouche Jr. and a self-styled "public health expert" -- all said they favored the death penalty, particularly in cases involving espionage.

Candidate Boyd A. Sweatt did not respond to the invitation to participate in the debate, League officials said.

U.S. Rep. Steny Hoyer (D) and Del. Albert Wynn (D-Prince George's) announced last week that they have joined forces to run a slate of candidates in the county's 25th District.

In forming what they call the "Democratic Alliance" ticket, Hoyer and Wynn have overcome the mutual uneasiness expressed on both sides earlier this year.

Wynn, who represents Largo, Kettering, Suitland, District Heights and Forestville, is seeking the state Senate seat being vacated by B.W. Mike Donovan.

He is running against Democratic Del. Jerry Perry.

The "alliance" ticket is an interesting example of the disjointed tickets being formed throughout Prince George's County this election year. Hoyer, for example, is supporting Baltimore Mayor William Donald Schaefer for governor, while Wynn is backing Schaefer's opponent, Attorney General Stephen H. Sachs.

Both candidates, however, are supporting lawyer Alex Williams for state's attorney.

Republican U.S. Senate candidate Richard Sullivan blasted two of his Democratic opponents this week, saying that their votes in Congress showed that they support legislation that "wrecked the nation's economy" during the Carter administration.

Sullivan said Reps. Barbara A. Mikulski and Michael D. Barnes, both candidates for the Senate, have voted against legislation that would make it easier for Marylanders to afford housing.

Sullivan said the two veteran representatives supported the economic policies of President Carter that gave the nation a 14 percent inflation rate and 18 percent mortgage rates.

The former chairman of the Easco Corp. said he stood squarely behind President Reagan's economic policies because "they've worked."

"Let's keep moving the economy forward," he added.

Barnes, in a statement released by his campaign office, said "The Reagan administration's economic policies have resulted in the largest deficits in the history of the world. They have turned our country into a debtor nation at the expense of public education, environmental protection and services for the handicapped and elderly.

Wendy Sherman, Mikulski's campaign manager, said the congresswoman would have no immediate comment to Sullivan's charges.

Sullivan said he did not take on Linda Chavez, his chief rival for the Republican nomination to replace retiring Sen. Charles McC. Mathias Jr. because "I'm very confident about the primary."From news services and staff reports