U.S. Rep. Joe Kennedy III, D-Mass., said in a Facebook post Monday that he’s weighing a challenge to incumbent U.S. Sen. Edward Markey, D-Mass., in the 2020 Democratic primary. Carolyn Kaster/Associated Press, file

BOSTON — Massachusetts Rep. Joe Kennedy III, a scion of one of America’s most storied political families, is taking steps to challenge Sen. Edward Markey in the 2020 Democratic primary, setting the stage for what could be a bitter intraparty battle split along generational lines.

Kennedy, 38, filed a statement of candidacy with the Federal Election Commission on Monday, though in a Facebook post he stopped short of formally declaring a run against the 73-year-old Markey, who is seeking a second full term in the Senate after spending decades representing the state in Congress.

“I am humbled by the words and actions of so many people supporting my potential candidacy. It means the world,” said Kennedy, a grandson of Robert F. Kennedy. “I plan to spend the next couple weeks talking to as many of you as I can, trying to figure out if this campaign is right for me and right for Massachusetts.”

Kennedy, who is in his fourth term in Congress, added that while some people have told him he should wait his turn, “I’m not sure this is a moment for waiting.”

U.S. Sen. Edward Markey, D-Mass., speaks during a news conference Monday in Boston. Markey, 73, is running for a second full term in the Senate after serving for more than three decades in the House. Elise Amendola/Associated Press

The potential for what would be a rare, serious challenge to an incumbent delivered a jolt felt beyond Massachusetts and has the potential to divert resources Democrats would rather spend loosening Republicans’ grip on the Senate in battleground states. As an incumbent, Markey can count on support from the Democratic Senate Campaign Committee, which is Senate Democrats’ political arm.

Kennedy is a rising star in the party who delivered Democrats’ rebuttal last year to President Trump’s State of the Union address. He had previously signaled his intention to run for re-election in his House district, which stretches from the western suburbs of Boston to southeastern Massachusetts.

But in recent weeks speculation about a possible primary challenge of Markey has ramped up considerably.

“I’m running hard on the issues the people of Massachusetts care most about,” Markey said Monday at an event in Boston, citing the immigration and environmental causes and stricter gun laws he’s long championed. “That’s all I’ve done, from the day I was given the honor to represent this state, and that’s what I’m going to continue to do.”

When asked if he would still run even if Kennedy officially entered the race, he replied without uttering the name of his potential Democratic rival: “I am going to crisscross this state and I am going to give it everything I got.”

Kennedy’s flirtation with a challenge comes a week after fellow Massachusetts Sen. Elizabeth Warren, one of the party’s leading presidential candidates, endorsed Markey.

“We need Ed Markey in the Senate now more than ever. And here’s why. Because he’s a leader. He’s a fighter. And he is a true progressive,” Warren said in a video offering her endorsement. “Ed is my friend and my partner in the Senate. I’ve got his back, and I know that he will always have yours. Let’s get Ed Markey re-elected to the Senate.”

Kennedy’s father, Joseph P. Kennedy II, served in the U.S. House, as did his cousin, Patrick Kennedy of Rhode Island. His two great-uncles, former President John F. Kennedy and Edward M. Kennedy, also served in the U.S. Senate representing Massachusetts.

Although Joe Kennedy is a vocal critic of Trump on issues including immigration, climate change and LGBT rights, a campaign against Markey would hardly amount to a challenge from the left.

In fact, Markey has been fortifying his own credentials with the more liberal wing of the Democratic Party. In February, he joined freshman U.S. Rep. Alexandria Ocasio-Cortez of New York in sponsoring the Green New Deal, a sweeping blueprint to transform the U.S. economy through combating climate change and creating jobs in renewable energy.

Neither campaign is lacking financial resources, either.

At the end of the last fundraising period, Kennedy reported having $4.2 million cash on hand in his campaign account, barely edging Markey, who posted just over $4 million, campaign finance records show.

Two lesser-known Democrats, workers’ rights lawyer Shannon Liss-Riordan and businessman and author Steve Pemberton, previously announced their intention to run against Markey in the September 2020 primary.