Democratic Rep. Joe Kennedy III is considering a 2020 Senate primary challenge against Massachusetts Sen. Edward Markey -- which could pit a longtime, well-known Democratic lawmaker against a scion of one of the nation's most famous political families.

Kennedy, the son of former Rep. Joseph Kennedy II, D-Mass., and grandson of Robert F. Kennedy -- the former U.S. senator from New York who served as attorney general in his brother John's administration -- wrote on Facebook on Monday that he is not mulling the decision "lightly," but wanted to share his thoughts with his supporters.

"Over the past few weeks I've begun to consider a run for the U.S. Senate," Kennedy said. "This isn't a decision I'm approaching lightly and -- to be completely candid -- I wasn't expecting to share my thoughts so soon. Family is my first consideration with any big decision like this," he said.

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"I haven't reached a decision yet -- that's the truth. I'm thinking about what I have to offer Massachusetts voters, what is most important in this political moment, and what kind of party Democrats need to be building for the future," he continued in the Facebook post.

In addition, the 38-year-old lawmaker said he has been humbled by the encouragement of supporters, but remarked that some people have advised him to wait his turn.

"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," he said.

"I've got some ideas on how to do that. And I don’t think our democratic process promises anyone a turn. What it does promise is the chance for anyone to earn it — if we think we have something to offer and are willing to put ourselves and our ideas out there."

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His potential opponent, Markey, has been active in Massachusetts politics for decades -- previously serving as a U.S. congressman from the Boston area for more than three decades.

Markey, 73, won his Senate seat in a 2013 special election held after then-Sen. John Kerry resigned after being nominated by ex-President Barack Obama to serve as secretary of state.

More recently, Markey has been a vocal proponent of climate change legislation. Earlier this year, he co-authored the Green New Deal with Rep. Alexandria Ocasio-Cortez, D-N.Y.

For his part, Kennedy was recently given the prominent role of offering the minority rebuttal to President Trump's State of the Union.

In his 2018 speech, the lawmaker called Trump a "bully" and claimed Russia was "knee-deep in our democracy."

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“Bullies may land a punch. They might leave a mark. But they have never, not once, in the history of our United States, managed to match the strength and spirit of a people united in defense of their future,” Kennedy said.

Following a report earlier this month that Kennedy has been mulling a bid for the seat, Markey’s campaign told The New York Times that he still plans to run for re-election, regardless of what Kennedy decides.

“Ed is not going anywhere,” Markey adviser Paul Tencher said. “He’s going to run, and he’s going to run no matter who is in this race.”

In July, Politico reported the existence of a Massachusetts telephone poll of a hypothetical Markey-Kennedy race. A "Jump in, Joe!" group of Kennedy supporters later launched a website and a Facebook group.

“He has energy, courage, integrity, and progressive ideas,” the website for the group said. “The United States Senate needs a bench of proactive, progressive leaders to counter the divisive tactics of Senator Mitch McConnell and his Republican Party. We believe the United States Senate needs Joe Kennedy III.”

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In Massachusetts' U.S. Senate elections of late, both incumbents have won handily over their Republican opponents.

Sen. Elizabeth Warren, D-Mass., won 60 percent of the vote against her 2018 Republican challenger, State Rep. Geoff Diehl.

One year after winning the special election to succeed Kerry's appointed replacement, former Sen. Mo Cowan, D-Mass., Markey defeated Hopkinton, Mass. lawmaker Brian Herr by a similar margin in 2014.

Fox News' Molly Line, Alex Pappas and the Associated Press contributed to this report.