Policy differences between Sen. Edward Markey and U.S. Rep. Joe Kennedy III, who is vying for Markey's seat, became clear in their first televised debate Tuesday night, sponsored by WGBH.

One such difference is on whether to ban outside money from their race. Kennedy wants to; Markey wants to allow many of the groups that support him — on labor, on the environmental movement and women's groups — to be allowed to buy ads.

Another issue: Afghanistan. Kennedy argued for bringing troops home. Markey proposed keeping some in Afghanistan to make sure the Taliban respect any eventual peace treaty.

At one point, Kennedy ridiculed Markey's decision to be the only senator to vote "present" on a resolution to allow former President Barack Obama's to use force against the Syrian government after it attacked its own people with chemical weapons.

"It's hard for me to understand when a 'present' vote is going to be a profile in courage," Kennedy said, taking a dig at Markey while at the same time, referencing a book by his great-uncle, former President John F. Kennedy.

Markey explained he wanted the administration to present all its intelligence before the full Senate took a vote.

Since many of their positions are so similar, Kennedy was asked by moderator Jim Braude, of WGBH, if he is simply an opportunist in challenging Markey for a safe Democratic seat in the Senate. Kennedy replied that progressive voting records don't matter, as long as Majority Leader Mitch McConnell controls the Senate and Democrats aren't able to pass legislation. He promised to use the job as a bully pulpit.

"You have to make sure that you're doing everything that you possibly can to restore power to a Democratic Party so that we can actually advance the causes we care about and protect the people we care about," Kennedy said.

"Has he been a good senator?" Braude asked.

Rep. Joe Kennedy III and Sen. Ed Markey square off in the first Senate primary debate on Tuesday in Boston. (Meredith Nierman/WGBH via AP, Pool)

"Absolutely, he's been a good senator, but Jim, this isn't about whether Sen. Markey has made an important contribution," Kennedy said. "Of course he has."

Markey defended his record as a champion for environmental causes.

"When I introduced the Green New Deal with [U.S. Rep.] Alexandria Ocasio-Cortez just one year ago, it transformed the way in which climate change is being debated in the United States and across the planet," Markey said.

He also spoke about his efforts to push for gun control laws.

Kennedy countered that Congress has not passed meaningful gun control legislation in decades.

"With Mitch McConnell saying look, what this is about is maintenance and access to power, and he is going to hold onto it with everything he's got," Kennedy said.

"Can I interrupt?" Markey broke in. "Honestly, that's just not accurate. The last meaningful gun safety legislation passed in the last week of December 2019, when my bill was actually signed by Donald Trump to begin research at the [Centers for Disease Control] that had been blocked for 25 years by the [National Rifle Association]."

The new law authorizes the CDC to conduct research on the public health hazards of guns.

After the debate, when asked what he would do that Markey has not done, Kennedy said policy matters, but argued a senator from Massachusetts should be doing much of his work outside the Senate.

"Look at the way that we've run this campaign," he said. "We did 13 town halls this month alone. Two of them were in Spanish — the first time any candidate's done town halls in another language. I was in nearly 20 states last cycle, campaigning for other colleagues and helping to flip the House of Representatives. I think the ability that comes with a Massachusetts Senate seat enables you to go around the country to build that bench, to help elect mayors and local school boards and state reps. and state senators."

Markey stood by his record.

"My bills are the law," he said. "My bills are what changes the debate in our country. That is who I am. That is what I have done with my career."

Throughout the night, it became clear that the two candidates hold similar views on the issues, but very different views on how a senator from Massachusetts should spend his time in the Senate of Mitch McConnell.

WBUR is co-sponsoring the next debate, in March in Springfield, as well as the last debate before the Sept. 1 primary.