U.S. Rep. Joe Kennedy III, a grandson of Robert F. Kennedy, is taking steps to challenge Sen. Edward Markey in the 2020 Democratic primary in what would be a rare, serious challenge to an incumbent.

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 and said he will take several factors into account, including his young family.

“I am humbled by the words and actions of so many people supporting my potential candidacy. It means the world,” the congressman wrote. “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.”

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

Kennedy is in his fourth term in the House and had previously indicated plans to seek reelection next year.

Markey, 73, is running for a second full term in the Senate after serving for more than three decades in the House.

Two of Kennedy’s great-uncles, John F. Kennedy and Edward M. Kennedy, served as senators from Massachusetts.

(AP)