Responding to speculation that he'll challenge U.S. Sen. Ed Markey in next year's Democratic primary, Congressman Joe Kennedy III on Monday said he is not sure that "this is a moment for waiting" but hasn't yet reached a decision about his 2020 plans.

Posting on Facebook at 10:55 a.m., Kennedy said he has "begun to consider a run for the U.S. Senate" over the past few weeks and is 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."

"I hear the folks who say I should wait my turn, but with due respect -- I'm not sure this is a moment for waiting. 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," Kennedy wrote. "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."

At 11:37 a.m. Monday, Kennedy filed a statement of organization with the Federal Election Commission creating a "Kennedy for Massachusetts" Senate campaign committee.

Kennedy's Congressional office at 11:43 a.m. sent out a public schedule advising a Tuesday visit to the Newton Fire Department.

Kennedy, the 38-year-old grandson of Robert F. Kennedy who was first elected to the House in 2012, has not held public events in recent days. The Facebook post is his first public comment on rumors of a run against Markey.

Describing himself as "humbled by the words and actions of so many people supporting my potential candidacy," he said he planned to "spend the next couple of weeks" talking to voters and "trying to figure out if this campaign is right for me and right for Massachusetts."

Markey, meanwhile, has spent the last several days announcing a blitz of endorsements, gathering an early slate of backers that includes U.S. Sen. Elizabeth Warren, NARAL Pro-Choice Massachusetts, the Sunrise Movement, the Coaition for Social Jsutice, and the League of Conservation Voters Action Fund.

Markey was first elected to the Senate in a 2013 special election after 37 years in the U.S. House. Markey did not face a Democratic primary challenger in 2014, and in the 2013 election defeated U.S. Rep. Stephen Lynch with 57 percent of the vote. Before that, he had last faced an opponent from within his own party in 2002.

So far this cycle Markey has at least two declared opponents from within his own party -- Shannon Liss-Riordan and Steve Pemberton. Kennedy's entrance in the race would shake up that field and clear the path for a slew of potential Congressional candidates interested in the seat he holds but wary of taking on an incumbent.

A late July telephone poll asked respondents about a potential Markey/Kennedy matchup, and about a primary field that consisted of Markey, Kennedy, Liss-Riordan, Pemberton, and Attorney General Maura Healey, who has not expressed an interest in running.

There's plenty of time for candidates to decide whether to run. May 5 is the deadline for candidates for Congress and statewide offices to submit their nomination papers to local officials.