U.S. Sen. Edward Markey and challenger Shannon Liss-Riordan sparred Sunday at a forum focused on climate and environmental issues. The debate, held at Stonehill College, delved into the candidates' perspectives on the Green New Deal, carbon emissions policies and Senate debate procedure.

But multiple attendees were focused on the candidate who was not present — Rep. Joe Kennedy III.

“To not be there really sends the message that it’s not a priority for him,” said Emerson Toomey at a rally for Sen. Markey before the forum. Toomey, 19, is a Northeastern University student and a volunteer with the Boston arm of the climate activist Sunrise Movement. She said the event represented an opportunity for policy-makers, advocates and curious voters to hear from those seeking the senate seat in next year’s election.

The Kennedy campaign maintains he would agree to a climate debate under different circumstances.

“The Congressman strongly believes this particular debate should be held in a frontline community most impacted by our country’s climate failures and that a 2020 date will ensure better voter attention on this critical issue, rather than the middle of Veteran’s Day weekend,” said Emily Kaufman in a statement Sunday. Kaufman added Kennedy “believes debates should be a collaborative effort between all campaigns, not organized by one candidate without meaningful input from others.”

Asked whether she thinks a debate in 2020 would ensure better attention from voters, Toomey said, “The climate crisis can’t wait, and I think it’s really unfortunate that he feels that it’s convenient to pick and choose when he can get involved with it.”

Liss-Riordan supporters also pointed to the missing candidate.

“It’s definitely going to help her,” said Vatsal Chokhshi, while standing outside of the venue before the event. “If a person [of that] stature is not here for the debate, what is he not saying?”

At the debate, Markey expressed support for changing the rules of the Senate filibuster — a tactic used to delay a vote on a bill by extending debate. The current procedure for breaking a filibuster requires 60 votes. Markey said he’d consider a change if Democrats do not seize control of the Senate and Republicans block votes.

“At that point, we will have to talk about changing the filibuster,” he said. His answer represented a changed stance on the issue, which his two challengers had already expressed support for. Liss-Riodan unconditionally favors eliminating the procedure.

The two also differed in their support of U.S. presidential candidate Sen. Elizabeth Warren’s proposal to immediately ban fracking.

Markey hedged, saying deploying wind and solar sources would eliminate the need for fracking. Meanwhile, Liss Riordan gave another unconditional “yes.”

Both support the Green New Deal — a congressional resolution for addressing climate change, which Markey co-sponsored — but Liss-Riordan criticized its lack of details.

“It is a broad document that speaks in terms of goals and policies, but doesn’t have any of the specifics about how we’re going to get there,” Liss-Riordan said.

In response, Markey characterized it as a reframing measure that would make climate change a congressional priority.

“We need a new agenda, that’s what the Green New Deal is. It’s meant to create a political revolution in our country that will change the way people view these issues,” he said.

The pair also differed on best methods for cutting greenhouse gas emissions and moving away from fossil fuels. Markey emphasized tighter fuel economy and electric efficiency standards, while Liss-Riordan argued for instituting a federal carbon tax.

Liss-Riordan challenged Markey on his effectiveness at passing bills. She pointed to his failed 2009 cap and trade bill, the American Clean Energy and Security Act, which died in the Senate despite the fact that Democrats held both chambers of Congress.

“Senator, you’ve been in Washington for a long time and we have not made the progress that we need to make. It is time to move beyond talking about these issues and actually getting things done,” she said.

Markey pointed to Democrats' recent gains in the Virginia legislature — Democrats last week won control of the House and Senate for the first time in a generation — and argued the political climate has shifted in favor of more progressive policies.

“What happened in Virginia is going to happen in our country,” he said. “I think that Republicans in our country should beware of whether or not they want to tie themselves to this fossil fuel agenda and risk losing their seats.”

Markey received loud applause from the audience, which Stonehill officials say contained about 300 registered attendees. The event was moderated by MSNBC political correspondent Steve Kornacki.

The election for the Senate seat is now less than a year away.