Townsend announced the plan at the Believe Day Festival at Druid Hill Park, where she gave a short speech to a largely African-American audience as the festival got under way.
Townsend's $30 million pledge would increase state funding for city drug treatment by about 62 percent.
Mayor Martin O'Malley welcomed the announcement.
"I think it's great. That would be a big, big help to Baltimore and Maryland in reducing drug addiction," he said.
O'Malley seriously considered running for governor in a primary against Townsend, and has frequently referred to a "vacuum of leadership" among at the top of Maryland's Democratic ticket.
Yesterday, however, he offered only support. "I've always said I'll support the Democratic nominee, and I've always said the nominee needs to make specific commitments like this one to win the election."
Townsend had been hearing the same message from other prominent city backers, such as state Sen. Clarence W. Blount and Rep. Elijah E. Cummings, who have been urging her to tell city voters exactly what she plans to do for them.
In fiscal year 2003, Baltimore is slated to receive $48 million, according to Townsend's office. Her proposal is to add new money over a three-year period, starting in fiscal year 2005.
She did not say how she would raise that money, or in what increments she would include it in each budget.
Townsend campaign spokesman Len Foxwell said those decisions would depend on the health of the economy.
Townsend was chairwoman of a state task force that studied Maryland's drug problems. It recommended in 2000 spending $200 million more over 10 years, statewide. Since the report came out, spending has increased by $56 million, Foxwell said.
"Kathleen's goal is to do as much of the rest as possible and maintain the pace to completely honor that commitment by 2012," he added. "This is clearly going to be a top priority of Kathleen's."
Rep. Robert L. Ehrlich Jr., a Republican who is challenging Townsend, also has frequently mentioned drug treatment money, particularly when campaigning among city voters, and says it is also "a major theme" of his campaign.
He has not issued a specific plan, but said drug money would be reflected in a "budget blueprint" he plans to release this month. He said it would be "irresponsible" for him to name a dollar figure before his staff had balanced that blueprint.
Ehrlich criticized Townsend for parceling out $30 million when the state faces a nearly $1 billion deficit. "We are really concerned about the numbers that are piling up under her watch," he said.
Both campaigns spent much of the day in Baltimore, riding around in trolleys at opposite ends of the city.
Townsend joined state Sen. Nathaniel J. McFadden and his slate of candidates in the 45th District, making stops at a Gardenville youth football game and several supermarkets in Northeast Baltimore.
Ehrlich walked the streets of the predominantly white neighborhood of Violetville in Southwest Baltimore, promoting Sen. Clarence M. Mitchell IV, who is leading the "Democrats for Ehrlich" campaign. Mitchell is running against Del. Verna L. Jones in the 44th District.
At house after house, Ehrlich grasped Mitchell by the shoulder, pointed at him and said to voters: "He's a personal friend of mine. This is a good guy. I'd really appreciate it if you would vote for him."
That was enough to convince Regina and Lawrence Lepus. Asked what they knew of Mitchell, they said, "We know he supports Ehrlich."
Sun staff writer Jackie Powder contributed to this article.