Rep. Joe Kennedy III (D-Mass.) said in a Facebook post on Monday that he's considering a run for Senate in Massachusetts, a decision that could set up a Democratic primary against Sen. Ed Markey (Mass.).

"Over the past few weeks I've begun to consider a run for the U.S. Senate. This isn't a decision I'm approaching lightly and -- to be completely candid -- I wasn't expecting to share my thoughts so soon," Kennedy said in a post to supporters.

Kennedy filed paperwork with the Federal Election Commission on Monday, allowing him to raise for a Senate bid.

Markey is up for reelection in 2020, and speculation that Kennedy could challenge his fellow Democrat has been mounting.

Sen. Elizabeth Warren (D-Mass.), the state's other senator, is currently running for president and her seat would also open up should she win that race. Her next Senate reelection would be in 2024.

A senior adviser to Markey's campaign told The New York Times that Markey was not considering stepping aside.

"Ed is not going anywhere," Paul Tencher said. "He's going to run, and he's going to run no matter who is in this race."

A group titled Jump in, Joe! is already pushing voters online to encourage Kennedy to launch a challenge.

"The congressman should run because our country is vulnerable, and he has demonstrated that he has the energy, courage, and progressive ideas to fight for the Commonwealth and put our nation on a more just course," the group said on its website.

Kennedy, currently in his fourth term in Congress, comes from a well-established political dynasty that has featured former President John F. Kennedy, former Sen. Ted Kennedy (D-Mass.), as well as former Attorney General Robert F. Kennedy.

Kennedy said that he is motivated to run because "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."

"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," he said in response to individuals who have advised him to wait.

Kennedy said that he has not yet reached a decision, and was considering a number of factors including the impact a campaign would have on his family. The congressman has two children under the age of four.

The young congressman is considered a rising star within the Democratic Party, and delivered the Democratic response to President Trump's State of the Union address in 2018.

- Updated at 11:46 a.m.