The 38-year-old Kennedy -- the grandson of Robert F. Kennedy – formally announced his candidacy at a community center in Boston after weeks of rumors that he was mulling entering the race.
"Donald Trump has forced a long overdue reckoning in America, and how we respond will say everything about who we are," Kennedy told supporters. "We have to take on the broken system that gave rise to him in the first place — the outdated structures and old rules, the everyday oppressions and injustices that hold our people back."
Should Kennedy win, he would be the fourth Kennedy to serve in the Senate, joining his grandfather and his great-uncles John F. Kennedy, the 35th president, and Edward Kennedy. Edward Kennedy was the last member of the prestigious political family to run for Senate, winning election in 1962.
The challenge to Markey, 73, is likely to leave local Democrats, including 2020 presidential hopeful and fellow Sen. Elizabeth Warren, D-Mass., with a difficult choice of who to back.
"Ed Markey is a proud and strong progressive champion for working families -- not just in Massachusetts but across the country," she said last week.
Markey, who joined thousands of young climate change activists who rallied in Boston on Friday as part of a global, youth-led day of environmental action, has the backing of many environmental activists. One of those groups — Environment Massachusetts — has vowed to raise $5 million to help Markey win re-election.
Kennedy positioned himself as a more pragmatic Democrat when he was first elected to Congress in 2012, but has since adopted a series of left-wing stances. He has backed impeachment proceedings against President Trump, "Medicare-for-all" and the Green New Deal. In 2018, Kennedy gave the Democratic response to Trump's State of the Union address.
Despite the blue-on-blue challenge, Kennedy has not yet moved to criticize Markey.
"Senator Markey is a good man," he said. "This is going to be a tough race."
But Markey responded on Saturday by challenging Kennedy, along with other primary opponents, to a debate on how to respond to the question of climate change -- an issue he has been advocating on for decades. He proposed it be held in November.
"I was very disappointed at the Democratic National Committee's refusal to hold a debate on climate change for our presidential candidates," Markey said in a video message. "So today I'm challenging Congressman Joe Kennedy, Shannon Liss-Riordan and Steve Pemberton to a climate change debate, and to do it very soon."
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“For the next generation, we can’t wait,” he said.
Liss-Riordan, a workers' rights lawyer, and Pemberton, a former senior executive at Walgreens, are also challenging Markey.
The Associated Press contributed to this report.