The five men vying to replace Secretary of State John F. Kerry in the U.S. Senate traded jabs Wednesday night at WCVB-TV studios in Needham, Mass., in the first televised debates of the campaign.

After each of a trio of Republicans—State Rep. Daniel B. Winslow, former U.S. Attorney Michael J. Sullivan, and former Navy Seal Gabriel E. Gomez—attempted to distinguish himself as the least unknown of the group, sparks flew in the second debate as U.S. Rep. Stephen F. Lynch fought to establish position in a Democratic race that many polls suggest will go to U.S. Rep. Edward J. Markey.

The most contentious issue of the Democratic debate was the Patient Protection and Affordable Care Act, the massive 2010 health care law which Markey supported and Lynch voted against.

Calling his support of the bill “the proudest vote of my career,” Markey hammered Lynch for voting nay. “Steve, when that vote came up, you were wrong when you were needed most,” he said.

But Lynch pushed back. Suggesting that the negotiations behind the health care law cast American patients as “hostages” to insurance companies, he lamented that Congress allowed insurance and medical corporations to acquire millions of new customers without lowering their prices.

“Now we’re in a tough spot,” Lynch said to Markey, citing the belief among some that parts of the bill, including a sales tax on medical devices, are flawed. “You’re supporting the repeal of sections of an act that you voted for.” Markey countered that he would only repeal aspects of the bill if doing so would not hurt the lower and middle classes. Lynch said he “would vote to fix” the bill rather than repealing it.

Despite his fluctuating record on the issue, Lynch took the offensive when the conversation turned to abortion. “You actually supported an amendment to overturn Roe v. Wade,” he reminded Markey, who voted to ban the procedure before changing his position in 1983.

In contrast, the GOP debate featured few such engagements. Gomez epitomized the less combative tone when, given an opportunity to ask a question of his Republican opponents, he instead chose to ask a rhetorical question of Markey and Lynch, who had not yet debated and were not in the studio. “I’m not here to ask questions of my fellow candidates to try to tear them up or put them on the spot,” he said.

Bringing the crowd and the candidates to laughter, moderator R. D. Sahl gave Gomez a friendly reminder. “You have to run against these fellows first,” he said.

The GOP candidates’ discussion of same-sex marriage proved testier. When Gomez and Sullivan delivered nearly identical answers supporting same-sex marriage and the repeal of the Defense of Marriage Act but essentially delegating the issue to the states, Winslow pounced.

“I think that this goes to the core values of who we are, equality and freedom,” he said. “Mr. Sullivan wants to have government intrude into the personal lives of people, something that is so personal: who they can get married to. I disagree with that.”

The debates were presented by the Boston Media Consortium. The Republican candidates will debate again Thursday in Springfield, and the Democrats will also debate in Springfield on April 18. Both parties will choose their nominees in closed primaries on April 30, to be followed by a special election on June 25.

—Staff writer Matthew Q. Clarida can be reached at clarida@college.harvard.edu. Follow him on Twitter @MattClarida.