Environmental activist and former Democratic presidential contender Tom Steyer on Monday threw his support behind Sen. Ed Markey (D-Mass.) in the incumbent's Senate primary race against challenger Rep. Joe Kennedy III (D-Mass.).

"For the last several decades, Ed’s been leading the fight in Congress to tackle our climate crisis while too many politicians on both sides of the aisle have sat on the sidelines," Steyer said in a statement first released to The Hill.

Steyer went on to say Markey is needed in the Senate to "combat climate injustice and create clean energy jobs."

Markey, a co-author of the Green New Deal legislation, praised Steyer's endorsement, calling him "one of the most influential voices" in the fight against climate change.

"He has organized Americans around the country to help position climate action and clean energy as top voting issues in this election, and his support of this movement is invaluable," Markey said.

The endorsement comes seven years after the Massachusetts senator denounced Steyer's involvement in his first Senate bid, in 2013. Markey's campaign argued at the time that Steyer's plan to spend a significant amount of money in the primary to defeat Markey's then-opponent, Rep. Stephen Lynch (D-Mass.), would violate Markey's commitment to the People's Pledge, which prevented him from accepting outside advertising. Steyer said he was backing Markey because of Lynch's support for the Keystone XL pipeline.

The People's Pledge has become an issue in 2020, with Kennedy pushing Markey to again sign it. Markey has refused to sign on, instead calling for a compromise in which outside groups would keep their messaging positive and make their donors public.

Kennedy rejected outside super PAC help earlier this month and called on Markey to take the same stance.

Kennedy responded to Steyer's endorsement on Monday by highlighting his own opposition to super PACs and calling on Markey to sign the People's Pledge.

“I have made it clear that I want to run this race without Super PACs, without dark money, and focused solely on the issues and on the merits of our candidacies,” Kennedy said in a statement. “That’s why, so far, I have rejected Super PAC money on my behalf. From day one of the campaign, I have asked Senator Markey to sign the same pledge he signed seven years earlier. But Senator Markey –– a so-called progressive champion –– has refused and already allowed dark money to infiltrate this race."

Steyer's endorsement on Monday comes as more money pours into the race ahead of the Sept. 1 primary. The Boston Globe first reported that an outside group, Environment Massachusetts, would spend $200,000 on print and digital advertisements in support of Markey.

The battle between the two progressives has divided Democrats on Capitol Hill. Markey has the support of a number of prominent figures including Senate Minority Leader Charles Schumer (D-N.Y.) and Sen. Joe Manchin (D-W.Va.) as well as Rep. Alexandria Ocasio-Cortez (D-N.Y.), co-author of the Green New Deal.

Kennedy has also scored a number of endorsements, including Mark Pocan (D-Wis.), co-chair of the House Progressive Caucus, and Reps. Raúl Grijalva (D-Ariz.) and Joaquin Castro (D-Texas).

Updated at 2:59 p.m.