Joe Kennedy announced on Saturday morning that he will contest the US Senate seat for Massachusetts held by Edward Markey, in what promises to be an intra-party fight exposing divisions in Democratic ranks.

The 38-year-old congressman is a scion of one of America’s most famous political families. He is a grandson of Robert F Kennedy, who was attorney general under his brother John F Kennedy, then a senator for New York and a presidential candidate before being assassinated, like his brother before him, in 1968.

John and Robert’s brother Ted was a prominent Massachusetts senator until his death in 2009.

Joe Kennedy made official his challenge to Markey, 73, a co-sponsor with Alexandria Ocasio-Cortez of the Green New Deal, on Saturday morning.

“This isn’t a time for waiting,” Kennedy wrote, “for sitting on the sidelines, or for playing by rules that don’t work any more. This is the fight of our lives, the fight of my generation – and I’m all in.”

In a video, he added: “In this moment, we refuse to accept the status quo that tells us big things aren’t possible. I’m running for Senate because this is the fight of our lives.”

The son of former Massachusetts congressman Joe Kennedy II will pose a serious challenge. A Boston Globe and Suffolk University poll released earlier this month found Kennedy leading by 14 points in a head-to-head contest.

Markey has been in Congress since 1976 and the Senate since 2013. In a statement anticipating Kennedy’s challenge, he said he would “run on all the issues I’ve been fighting for today and into the future”, which he listed as “climate change, income inequality, a woman’s right to choose and on gun safety legislation.

“I am going to continue those fights into this campaign and I will do so every day that I’m out there crisscrossing the state of Massachusetts and fighting for the people of our state.”

Kennedy and Markey are not far apart ideologically but the challenger sees a national Democratic party with more energetic leadership, standing up to Republicans who have chosen to back Donald Trump.

“This about leadership,” a Democratic operative familiar with Kennedy’s thinking told the Guardian on Saturday. “It’s why he was asked to offer the party’s response to Trump’s state of the union address [in 2018], and should be seen in the context of the post-2016 election [era].”

The challenge for Kennedy will be to articulate a compelling reason, other than his bloodline, for seeking to knock Markey out. The congressman planned a seven-stop tour of Massachusetts on Saturday, meeting figures from Democratic state leadership.

On Friday, Salem mayor Kim Driscoll co-authored an op-ed in CommonWealth magazine in which she explained her endorsement of the congressman.

“As mayors, our highest priority when it comes to partners in Congress is a leader who shows up. And Joe shows up for Massachusetts,” she wrote.

His challenge will also invite criticism that he is embarking on an expensive and divisive fight at a time when the party should be united in seeking to unseat Trump and secure a majority in the Senate.

Markey’s supporters include his fellow Massachusetts senator Elizabeth Warren, who told reporters last month he had been “a great partner in the Senate”. But the presidential hopeful also acknowledged Kennedy, noting he had once been her student.

“I worked with Joe before either one of us was in politics and have worked with him ever since he’s been in Congress,” she said. “He’s really an amazing person.”

Markey has the support of the Senate majority leader, Chuck Schumer, and Ocasio-Cortez, the New York congresswoman who dominates the progressive wing of the party. He has also lined up more than 100 state legislators, many of whom point to his leadership on climate change.

“We are lucky to have one of the nation’s strongest climate champions, Ed Markey, representing Massachusetts in the US Senate,” Environment Massachusetts state director Ben Hellerstein said on Friday, saying the group would put $5m forward to support his reelection campaign.

“Voters need to know what Ed Markey has done throughout decades of public service to advance renewable energy, clean water, clean air, and the protection of our public lands. We intend to tell his story.”

The Kennedy family has never lost a statewide election. In his announcement on Saturday, he said: “It’s extremely meaningful to me to be able to do this here, where in 1848 a few steps from where this building is today, my father’s family first arrived here in this country.”

Some Massachusetts Democrats have been unwilling to be drawn on their choice. Congresswoman Ayanna Pressley, a member of “the Squad”, has said she looks forward to working with Markey on environmental and gun control issues and with Kennedy on mental health.

“I think primaries are healthy,” she said.

Kennedy said: “The race is going to be decided by the people of Massachusetts.”