Three Maryland Democrats running for governor next year sought to convince a labor union of their bona fides on Monday by backing priorities including greater use of collective bargaining agreements on state construction projects.

Del. Heather R. Mizeur (D-Montgomery), the first to appear before the Maryland State and Washington, D.C. Building and Construction Trades Council, told the group that all three contenders would promise to care about its issues. But she said union members should focus on this question: “Who has proven it, time and time again?”

Mizeur, Attorney General Douglas F. Gansler (D) and Lt. Gov. Anthony G. Brown (D) all suggested that they were the answer.

Gansler, who plans to formally announce his bid for governor Tuesday in Rockville, was the most aggressive in making the case for so-called project labor agreements, under which the state requires contractors to negotiate worker conditions through a union.

He said the administration of Gov. Martin O’Malley (D) should have been more aggressive in adopting such agreements, saying only one had been put in place in the past seven years and it had been subject to litigation.

“The problem is, from the top, nobody’s pushed it. We haven’t had them,” Gansler said.

He promised to seek legislation to foster more agreements in the future.

Mizeur said she pushed to have project labor agreements written into a bill this year boosting school construction in Baltimore. But she said her colleagues balked for fear it could undermine support for the plan.

“We decided to hold off, wait for a Mizeur administration where we get these things done,” she said.

During his remarks, Brown also pledged to boost spending on public school construction, which would create more jobs, he said. He cited his work on expanding public-private partnerships, such as the proposed light-rail Purple Line.

And without mentioning them by name, Brown chided his opponents for not backing recent job-creating legislation.

Gansler did not support a bill this year that funded new transportation projects by raising the gas tax, and he was silent last year on legislation that authorized a new casino in Prince George’s County. Mizeur actively worked against that bill.

During her appearance, Mizeur said she was perfectly up front with the union about her opposition, arguing that Maryland should focus on more cutting-edge industries.

Mizeur also relayed how central labor issues had been to her upbringing as the daughter of a union welder. She shared how her application to be a Truman Scholar two decades ago demonstrated her early interest in making conditions better for working people.

And she promised she would be a different kind of governor if elected next year.

“I’m going in there to blow the doors off the place,” Mizeur said.

Gansler called himself a “friend of labor” and touted his support for raising Maryland’s minimum wage from $7.25 to more than $10. He also relayed that he would like to cut the state’s corporate income tax rate from 8.25 percent to 6 percent, a move some in his party view warily.

“I don’t think being pro-business means being pro-management,” Gansler said. “It means pro-jobs.”

The same labor group also heard Monday from all four Democrats who have said they are running next year for attorney general: Sen. Brian E. Frosh (D-Montgomery), Del. C. William Frick (D-Montgomery), Del. Aisha N. Braveboy (D-Prince George’s) and Del. Jon S. Cardin (D-Baltimore County).