Rep. Joe Kennedy to announce primary challenge to Sen. Ed Markey

Rep. Joe Kennedy III, D-Mass., the grandson of Robert F. Kennedy, will formally announce a 2020 primary challenge to Sen. Ed Markey on Saturday in Boston, according to a source familiar with his plans, before kicking off a statewide tour to promote his candidacy for the Democratic nomination.

He informed Markey of his decision Wednesday, according to another source familiar with their conversation.

Kennedy's intentions were first reported by the Boston Globe -- teeing up one of the cycle's marquee intraparty clashes.

Within a couple of hours, Markey's campaign insisted he is the "right choice" in a statement Wednesday.

"In 2013, Ed Markey asked voters to send him to the U.S. Senate to fight for the people of Massachusetts," said John Walsh, Markey's senior campaign adviser. "Since then, he has fought on the front lines to show them they were right. ... Elections are about choices, and Ed looks forward to spending the next 14 months campaigning hard every day to show the people of the Commonwealth why he’s the right choice."

Walsh also made clear Markey is ready for any political fight ahead, asserting, "Now he wants to continue that leadership on the issues that matter most -- climate change, income inequality, gun reform, universal health care, reproductive freedom, and immigrant rights."

The highly anticipated primary will pit the young scion of the Massachusetts political dynasty against a veteran progressive who has spent 43 years in Congress -- and has no plans to give up the Senate seat he waited decades to fill.

Markey, 73, a co-author of the Green New Deal, has spent months preparing for a possible challenge, and has been endorsed by Rep. Alexandria Ocasio-Cortez and Sen. Elizabeth Warren, who taught Kennedy at Harvard Law School, back in February.

In response to the expected announcement, the presidential contender told reporters on Capitol Hill Wednesday night, "I endorsed Ed Markey last February. I couldn't ask for a better partner in the United States Senate. Joe, I have known Joe since before he got into politics. He was my student, he and his wife Lauren met in my class, and I have nothing but good things to say about him."

When pressed if Kennedy's bid is "ill-advised" by a reporter, Warren responded, "I have no criticism."

Kennedy, 38, has echoed his former professor in recent interviews about his potential candidacy and pitch to Massachusetts primary voters.

"This is about standing up to Donald Trump every single day, but it's also about addressing the failures and fissures and the way that this, our government, our society has been broken long before Trump got in office," he recently told reporters. "That's going to require, I believe, a different approach and new ideas in order to tackle."

Markey also faces several lesser-known Democratic primary challengers who have already declared in the race, including labor attorney Shannon Liss-Riordan and businessman Steve Pemberton.

ABC News' Cheyenne Haslett contributed to this report.