WITH THE TOUGH decision to give up his House seat and challenge an incumbent Democratic senator behind him, Joe Kennedy now faces an equally tough question: Why is he running?

The scion of America’s first family of politics is still struggling to frame a succinct answer.

“Because our system is broken,” he said at the start of an interview on Thursday night. “And I think the moment we’re in demands that people do everything they can to try to fix it. And that’s making sure that the voices out there that feel like they are left out, cut out, taken advantage of, looked over, not part of that system — that they’ve got a senator out there that’s going to meet them where they are, that’s going to show up, that’s going to champion those voices.”

For more than 20 minutes, Kennedy held forth amiably at a live recording of The Horse Race, the weekly podcast produced by the MassINC Polling Group. But when pressed several times with variations of the question asking why he’s taking on incumbent liberal Ed Markey, the response from the 39-year-old congressman was more parry than punch. 

Asked whether his run was another sign that the era of wait-your-turn-politics is over, Kennedy went into a long riff about his family, emphasizing that he’s his own man, while also proud of the contributions of Kennedys before him. Asked about concerns that he’s taking on a leading voice of climate-change policy, Kennedy cited his own solid record on environmental issues. 

Co-host Stephanie Murray of Politico pointed out that his campaign is not exactly in the mold of insurgent challengers Ayanna Pressley and Alexandria Ocasio-Cortez. “You’re already a member of Congress. You’re a Kennedy. So what club here needs breaking into?” she asked.

Kennedy’s comeback may have been the best answer possible to a practically no-win question. 

“I think the seat should go to whoever the people of Massachusetts believe the seat should go to,” he said. “I think the great thing about democracy is people can step forward and run for it.” 

Part of the difficulty with the question of why he’s running now is that the real reason may be one Kennedy is unlikely to share publicly — that Markey seems uniquely vulnerable for an incumbent liberal senator who has spent decades in office in a decidedly blue state. 

A veteran of more than 40 years in Congress, Markey was only elected to the Senate six years ago after serving 37 years in the House representing a swath of suburbs north of Boston.

In June, a Suffolk University/Boston Globe poll showed that 36 percent of Massachusetts voters had never heard of Markey or had no opinion of him. In a September Suffolk poll of likely Democratic primary voters that number drops to 24 percent, but it’s hardly a reassuring sign of strength. It helps explain why the same poll showed Kennedy with a 14-point lead in a head-to-head matchup with Markey.

Issue-focused Democrats may be drawn to Markey’s leadership on issues like climate change or telecommunications policy, but he has not made a particularly strong statewide impression.

That relieves Kennedy of some of the pressure to explain why he’s challenging a fellow Democrat. Indeed, he may be able to treat the campaign more like an open race than a referendum on a well-known incumbent.

Seen in that light, the outlines of a campaign message seem clearer, particularly when Kennedy talks about reaching out to disaffected voters, including Donald Trump supporters, who feel let down by the ways of Washington. Kennedy said he’s aiming to “make sure that people that feel that they are left out, or looked over, or taken advantage of are brought back into the system.” 

While Markey is riding a wave of support tied to his cosponsorship of the Green New Deal, Kennedy’s message tilts more to themes of the original New Deal. He waves off the idea that voters are simply getting a newer-generation incarnation of one of his famous forebears. But his language carries distinct echoes of his grandfather, Robert F. Kennedy, who became a tireless voice for those at society’s margins and had a cross-racial appeal to the Democratic Party’s traditional base at a time when it was fracturing.

“Kennedy achieved a remarkable political coalition in time of strong political antagonism,” wrote Richard Kahlenberg in an essay last year titled “The Inclusive Populism of Robert F. Kennedy.” “He was a liberal without the elitism and a populist without the racism.”

Joe Kennedy seems to be unspooling some version of that. 

Kennedy may have been at his best when asked about the criticism that his challenge will divert resources that could be better used trying to flip seats to Democrats or hold them in close contests. 

“That’s bananas,” he said.

Kennedy went on to talk about energizing new people to work on campaigns and how that can build party strength, not sap it. He said his campaign recently helped bring 100 volunteers to New Hampshire to canvass for Elizabeth Warren. 

“The fact that you get more people involved,” he said. “How is that anything but a good thing?”