Two of Lt. Gov. Anthony G. Brown’s rivals in Maryland’s Democratic primary for governor questioned his ability Tuesday to deliver the planned Purple Line light-rail project in the Washington suburbs given the difficulties that have occurred on Brown’s watch with the state’s online health insurance exchange.

“Why do we believe that if we put the same people in charge of a similar project that is complex, untested and challenging to implement that we’ll end up with a different result?” Del. Heather R. Mizeur (D-Montgomery) asked during a gubernatorial forum in Silver Spring devoted to transportation issues.

Attorney General Douglas F. Gansler (D) also sharply questioned Brown’s leadership abilities during the forum, which was sponsored by a Purple Line advocacy group. Afterward, Gansler said to reporters: “These are the same people who couldn’t get a Web site going, so how do you imagine them getting the trains running?”

Brown (D), who emerged as the clear Democratic frontrunner in a poll released Tuesday by The Washington Post, did not attend the event because of a scheduling conflict.

He was represented by his running mate, Howard County Executive Ken Ulman (D), who sought to assure the audience that “the Purple Line is going to get built.” Ulman also told reporters afterward that he is “very proud of the lieutenant governor’s role in so many significant areas.”

Gansler has sought for months to blame Brown for the technological glitches and other problems with Maryland’s health insurance exchange that have hindered enrollment since its Oct. 1 debut. Though Brown did not oversee the day-to-day activities of the exchange, he was tasked by Gov. Martin O’Malley (D) with implementing federal health care reforms in Maryland.

Brown has also played a prominent role in the state’s decision to pursue private companies to design, build, operate and help pay for the $2.2 billion Purple Line, which would run between Bethesda and New Carrollton.It would be one of the broadest public-private partnerships of any U.S. transit project and the first of its kind for a Maryland transit line.

Because there are so many unanswered questions, the state will need a strong leader to make sure that such an arrangement is the proper way to go about getting the Purple Line built, Mizeur said.

Gansler said the state also needs to prepare a “Plan B” in case the state does not receive $900 million in highly competitive federal transit funding that it is counting on to help construct the planned 16-mile line.

Tuesday night’s forum was sponsored by the group Purple Line NOW! Besides the three Democratic candidates for governor, Republican hopeful Charles Lollar, a Charles County businessman, also participated.

Katherine Shaver contributed to this story.