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Despite the strong support for Mr. Markey, Mary Anne Marsh, a Democratic consultant, said that Mr. Lynch is hold his own and his opposition to the health care law has not torpedoed his campaign.

“This is Ed Markey’s to lose, but he is up against Steve Lynch, who is a master of defying expectations,” Ms. Marsh. “I think Lynch, so far, is doing a really quite a good job of trying to explain his stance on some of these issues, whether it is Affordable Care Act, or abortion, that in a Democratic primary in Massachusetts would normally be a real hurdle for him. He is doing a really pretty impressive job of explaining those positions to voters.”

Mr. Callahan and Mr. Kelly said that they understand, and respect, that Mr. Lynch opposed the health care law because he was concerned that it would take money out of their pockets.

“Even though it was an unpopular position to take in many circles, we think that really helps us define who Stephen Lynch is: someone who will stand up for what is right,” Mr. Kelly said.