SEEKONK, Mass. (WPRI) — Incumbent U.S. Sen. Ed Markey and challenger Congressman Joe Kennedy III faced off Tuesday night in their first televised Democratic primary debate, touching on a wide variety of issues during an hour-long exchange.

The event was hosted by public television station WGBH at its studio in Brighton. Here are five key takeaways from the debate.

Ed Markey and Joe Kennedy agree on a lot of key issues.

The campaigns don’t like to hear it, but the average primary voter isn’t going to see a lot of daylight between these two liberal Democrats on the big issues of the day, from guns and immigration to the environment. Even on thorny topics like eliminating private health insurance, they found themselves defending the same side of the argument.

Markey made sure to drive that point home, using one of his first comments in the debate to remind voters that he has partnered with Congresswoman Alexandria Ocasio-Cortez on the Green New Deal legislation — a key reason some progressives have rallied to his side and attacked Kennedy as entitled. He mocked Kennedy’s suggestion that no major environmental legislation has passed since the Nixon administration by highlighting a law he passed in 2007.

Still, there are differences between the two. Markey’s positions have evolved during his four decades in Congress — on abortion, for example, he has shifted from pro-life to pro-choice. But when it comes to how either man would vote in today’s hyperpolarized U.S. Senate, there’s little doubt each one would be a reliable supporter of the national Democratic Party agenda.

Kennedy is vague on why voters should dump Markey.

Since Markey’s current positions are generally in line with the preferences of Democratic primary voters, Kennedy can’t easily argue that his votes are a problem in the same way that he might if he were running against a Republican. Kennedy even described Markey as “a good senator” at one point — raising the implicit question of why, then, Massachusetts needs a new one.

Thus Kennedy is relying on broader arguments about why Massachusetts voters should dump the incumbent and elect him — although he generally avoids spelling out the most obvious one, that Markey is 73 years old and a congressional lifer, while Kennedy is only 39 and has been on Capitol Hill for less than a decade. (A few of his relatives had some experience there, of course.)

Kennedy argued that the rise of President Trump and his policy agenda has fundamentally altered the stakes in Washington, requiring Massachusetts to send a new and more energetic individual to make the most effective use of the Bay State’s second Senate seat. He also suggested again that Markey doesn’t spend enough time connecting with his constituents back home after so many years in Congress.

While “filing legislation and voting the right way is a critical part of this job,” Kennedy said, “You’ve got to have candidates that are going to, and senators that are going to, leverage every ounce of power that comes with a Massachusetts Senate seat.”

Markey disputes all of that, and can point to the support of Democratic activists who gave him the edge in last weekend’s opening convention caucuses. Still, as pundits like the University of Massachusetts Dartmouth’ Shannon Jenkins quickly noted, Kennedy may not need a more compelling answer to defeat Markey in the Sept. 1 primary considering he is already the frontrunner and financial leader in the race.

Markey’s foreign policy record is a vulnerability.

While Markey has spent more than 40 years in Congress, he has only spent seven of those representing the entire state; previously he was a congressman representing the district that includes his hometown of Malden. Markey’s aides acknowledge the primary is requiring him to effectively reintroduce himself to voters who have no strong impression of him — something he sought to do in the debate by mentioning his humble upbringing, a subtle contrast to Kennedy.

Yet the challenge with having such a long tenure on Capitol Hill is it also leaves a long trail of votes that aren’t always easy to defend in hindsight, and the debate’s exchange on foreign policy illustrated that.

Like many in his generation, Kennedy’s views on foreign policy were heavily colored by the Iraq war, and he has generally taken dovish positions as a House lawmaker. But Markey voted in favor of that war back in 2002 — unlike many of his fellow Democrats, including Kennedy’s late great-uncle U.S. Sen. Ted Kennedy, who led opposition to the conflict at the time.

“I regret that vote. It was a mistake,” Markey said during the debate, putting the blame on President George W. Bush and Defense Secretary Donald Rumsfeld for misleading him and other Democratic supporters of the war.

Markey was on similarly uncomfortable ground when he defended voting “present” on a resolution to let President Obama use force in Syria — he suggested he was waiting for more information, but Kennedy dismissed that as no “profile in courage.” Markey also expressed more hesitation than Kennedy about withdrawing U.S. troops from Afghanistan, expressing fear about the potential for destabilizing the country.

The ‘People’s Pledge’ debate is a flashpoint.

Analysts question how much people care about so-called “process” disputes between candidates over campaigning mechanics, but Kennedy and his aides are trying to make primary voters care about Markey’s rejection of a so-called “People’s Pledge” that would bar outside spending in the race.

The idea, pioneered in the 2012 U.S. Senate race between Elizabeth Warren and Scott Brown, involves each campaign agreeing to effectively pay a penalty if outside groups come in and spend money on its behalf — an effort to neutralize so-called “dark money” groups. Markey agreed to People’s Pledges in his 2013 and 2014 Senate races, but doesn’t want one this time.

Markey’s argument: the Trump presidency has changed the rules of the game, and therefore outside progressive groups should be allowed to spend money so long as they only run positive ads. “We need to have a new modern People’s Pledge, not an old one – one for the Trump era that allows for positive voices to speak,” he said.

Kennedy has pilloried Markey over the People’s Pledge for months, and did so again in Tuesday’s debate. “That is an exception that swallows the rule,” he said. “Look, what we have asked is – it was good enough for Senator Markey in 2013, it was good enough in 2014 when he championed it, when Massachusetts set the standard for keeping dark money out. It’s the same pledge now.”

The subtext: Kennedy is independently wealthy and raising more money than Markey so he’ll be fine without outside groups, while the incumbent is expecting an influx of cash from environmental groups who want to support him.

There’s a long way to go before voters finally have their say.

With the Senate primary still more than six months away, Tuesday night’s clash could feel like a distant memory by the time Massachusetts Democrats finally go to the polls to choose between Markey and Kennedy.

Neither candidate has aired any TV ads, nor would they be expected to until the election is closer and voters are starting to zero in. Multiple additional televised debates are being scheduled in the coming months, offering voters more opportunities to size up the differences between the two. And you never know what new issues will bubble up in the coming months.

Still, the first debate offered a solid introduction to the strengths and weaknesses of the two candidates — and a good indication of the likely battle terrain over the coming months.

Ted Nesi (tnesi@wpri.com) is WPRI 12’s politics and business editor and a Target 12 investigative reporter. He is a weekly panelist on Newsmakers and hosts Executive Suite. Follow him on Twitter and Facebook