"I've known Ed Markey for a long time. When he gets his Irish up, this is the Ed Markey you're going to see," said Larry DiCara, a former Boston city councilor who volunteered on Markey's 1976 congressional campaign. "I suspect Joe has been surprised at how many people are sticking with Eddie. This is not gonna be one of those steamroller situations as people may have suggested before."

Early polling put Markey behind Kennedy by 14 points in a head-to-head match-up, leading some pols to privately speculate that Markey may retire rather than risk losing his seat.

But the opposite has occurred.

On the morning of Kennedy's campaign launch for Senate, Markey pulled the classic challenger’s ploy — he called on Kennedy and his two lesser-known rivals, businessman Steve Pemberton and attorney Shannon Liss-Riordan, to join him in a climate change debate.

While Liss-Riordan and Pemberton accepted the debate invitation, Kennedy did not — Markey, after all, co-authored the Green New Deal and has put climate change front-and-center in his Senate campaign.

Instead, Kennedy's aides privately suggested the campaigns meet to discuss the framework for future debates. Even behind-the-scenes, Markey’s campaign took an aggressive posture.

"Three of the candidates have AGREED to correct the mistake of the national party by holding a stand-alone climate debate first and soon. The proposal reflects the urgency of the climate crisis,” Markey campaign manager John Walsh wrote in response to Kennedy's team, according to emails obtained by POLITICO. “Ed, Shannon, and Steve have agreed to a climate debate … Does Congressman Kennedy agree to join us, or do you continue to reject the principle and urgency of a stand-alone climate debate to be held first?”

Michael Goldman, a political consultant who advised Kennedy's father, former Rep. Joe Kennedy II, and grew up with Markey in Malden, said Markey’s forceful approach is a necessity.

"They're not blind to where they are," said Goldman. "He's got to run the insurgent campaign because he is the underdog. That's the whole key to this race. The incumbent has to run as an insurgent. He has to be the aggressor. It turns everything on its head."

Even though he's been in Congress since the Gerald Ford presidency, Markey does not have all the advantages an incumbent usually enjoys. Kennedy holds a double-digit lead in the polls, and Markey's name recognition and public approval ratings hover around 40 percent — less than ideal. Additionally, Markey and Kennedy are virtually tied when it comes to money. Both men are starting with a little over $4 million in their campaign accounts.

"Every poll says we're behind, I accept that. We're starting from behind, but we have the best candidate," said Walsh, the former chair of the state Democratic Party, who joined Markey’s campaign in late August.

Kennedy drew an early distinction in the race around corporate PAC money but Markey has sought to use it against him. Kennedy first pledged to stop taking that kind of cash several weeks ago, and criticized Markey for not doing the same. But Markey's campaign says the senator also stopped taking corporate donations last month — he just didn't announce it.

Then Markey turned the tables. He hit back at Kennedy's criticism with a 30-second video featuring a clip of Kennedy pledging not to take corporate PAC money, followed by a list of corporate PAC donations Kennedy accepted before he took the pledge. The list was superimposed over a grainy photo of the congressman.

"I put it out just to set the record straight so that it would be very clear, in fact, the real story about that issue," Markey told reporters at a picket line in Lawrence, Mass. on Saturday. Both he and Kennedy spoke at the event.

Asked whether the video was a fair attack, Kennedy simply said he stands by his record. He also called on Markey to take a People's Pledge, a pact to limit outside spending in the Senate race. Markey has not answered that call, and challengers Pemberton and Liss-Riordan have suggested Kennedy's call is opportunistic given the congressman’s fundraising prospects as a member of the Kennedy family.

"I stand by my record on this. That's up to him. I rejected corporate PAC money a while ago," Kennedy said at the strike on Saturday. "I would hope that all candidates in this race decide to abide by the People's Pledge to keep super PAC money and dark money out of this process."

Markey's campaign dismissed the idea that the senator is taking the fight to Kennedy because of his position in the polls.

"This is not, 'We're behind and we're hitting Joe Kennedy.' That's not the kind of campaign that Ed wants or that I seek," said Walsh, Markey's campaign manager.

"But it's equally true that we're not going to roll over," Walsh continued. "If someone's gonna attack Ed Markey, we're gonna respond."

The campaigns will gather Wednesday to hash out a debate framework for the next 12 months, before primary voters head to the polls next September. And while many dynamics in the race have been turned upside down, some conventions still old true, said Goldman, the consultant.

"I guarantee you that this is not a situation where Joe Kennedy is ducking debates and won't debate him. Of course they're going to debate because [Kennedy] is actually the insurgent. So the real cleverness of this is Ed Markey knows that Joe Kennedy wants to stand on a stage with him," Goldman said.