"I am here to announce I am gonna be a candidate for the United States Senate," said Rep. Joe Kennedy III.
In a tweet, Rep. Kennedy wrote, "This isn't a time for waiting, for sitting on the sidelines, or for playing by rules that don't work anymore. This is the fight of our lives, the fight of my generation — and I'm all in."
Markey may be hard to unseat. He spent 37 years in the House before he was elected to the Senate in a special election in 2013. Fellow Massachusetts Sen. Elizabeth Warren, a presidential hopeful, already formally endorsed Markey earlier this year.
In a Facebook post in August, Kennedy seemingly responded to critics questioning why he would challenge a well-liked Democratic incumbent. He said: "I hear the folks who say I should wait my turn, but with due respect — I'm not sure this is a moment for waiting. Our system has been letting down a lot of people for a long time, and we can't fix it if we don't challenge it."
Soon after Kennedy's announcement, Markey called on Kennedy and two other Democratic primary challengers to agree to a debate "very soon" that would focus on climate change.
Additional reporting from Newsy affiliate CNN.