BOSTON (AP) — Massachusetts U.S. Rep. Joe Kennedy III, a scion of one of America's most storied political families, is set to announce he will challenge U.S. Sen. Edward Markey in the state's Democratic primary in 2020.

A person with knowledge of Kennedy's plans told The Associated Press that Kennedy will formally make the announcement Saturday. The person wasn't authorized to preempt Kennedy's announcement and spoke Wednesday on condition of anonymity.

The 38-year-old grandson of Robert Kennedy has been quietly laying down the foundation of a run, building up his staff and formally announcing his interest in the race by filing preliminary paperwork with the Federal Election Commission last month.

"I don't think primaries are something that people should shy away from," Kennedy told reporters at the state Democratic convention last Saturday. "The idea behind it is that every seat, my own included, the one that I currently occupy as a member of the House of Representatives, it's up every two years. It's a two-year term. You have to go out and make that case to voters every two years."

Kennedy has shied away from directly criticizing Markey, calling him "a good man."

Markey, who's already facing two lesser-known challengers, has said he's ready to take on anyone — even Kennedy.

"I run every day on the issues that I've been fighting for throughout my career and that I'm continuing to fight for right now on the floor of the Senate. That's women's reproductive rights, climate change, gun safety laws, income inequality — and I'm going to continue to campaign on those issues," the 73-year-old Markey said at the same convention. "It's been the core of my agenda."

Kennedy is the latest in a long line of members of America's most celebrated political clan to seek elected office — most famously his uncle President John F. Kennedy, felled by an assassin's bullet in 1963.

Others include his father, former U.S. Rep. Joseph Kennedy; his grandfather Robert F. Kennedy, who was JFK's attorney general and was a senator running for the Democratic presidential nomination when he was slain in 1968; his uncle Sen. Edward M. Kennedy, who died in 2009; former U.S. Rep. Patrick Kennedy II, a son of Edward Kennedy; and his aunt Kathleen Kennedy Townsend, who served two terms as Maryland's lieutenant governor.

A senior campaign adviser to Markey said in a statement Wednesday that the incumbent is up for the political battles ahead.

"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," said John Walsh, a longtime political Democratic operative in Massachusetts.

Given his political pedigree, Kennedy has been seen as a rising star in the party. In 2018, Kennedy was tapped to deliver the Democratic response to President Donald Trump's State of the Union address.

A Kennedy-Markey contest will put more than a few high-profile Democrats in an awkward position — most notably White House hopeful Elizabeth Warren.

Warren has worked with Markey in the Senate and taught Kennedy at Harvard Law School. She formally endorsed Markey before Kennedy floated the idea of a challenge to Markey.

"I endorsed Sen. Markey back in February. I couldn't ask for a better partner in the Senate than Ed Markey. He is a good friend," Warren said. "Joe Kennedy is also a good friend. I have worked with him since he was a student of mine; both he and his wife were my students. I have worked with him as a congressman. I have nothing but the highest respect for him. And I have no criticism."

Kennedy has tried to position himself as more of a pragmatist than those on the left of his party.

Although he's adopted many of the causes driving the party's liberal wing — Kennedy has called for Congress to initiate impeachment efforts against Trump and has backed a "Medicare for All" bill in the House — he's also tried to carve out his own path.

In January 2017, as many Democrats were still reeling from Trump's win, Kennedy — first elected to Congress in 2012 — suggested that party leaders should be listening better to the economic worries of Democratic voters who bolted the party for Trump, saying that not taking the time to understand those voters would be folly.

He also argued that Democrats — then in the minority in the House — had to try to cut the best deals they could with Republicans.

"You've got to fight, but you've got to also try to move an agenda forward," he said at the time. "If you're just out there screaming and yelling, there are people out there who need help and need help now and they deserve progress, too."

Kennedy has also spoken frequently about what he calls "moral capitalism" — a less politically fraught term than "socialism" but one that has become central to his political worldview in the Trump era.

Markey is a formable opponent. He served for decades in the House before joining the Senate in 2013.

Markey has been quick to point out his endorsement by Democratic U.S. Rep. Alexandria Ocasio-Cortez of New York. The two have worked to push for the Green New Deal initiative.

"The Green New Deal will be the greatest force for blue-collar job creation in a generation," Markey has said.

Markey has also called for the House to begin an impeachment inquiry into Trump.

Kennedy and Markey have worked together at times.

Earlier this year the two reintroduced a bill that would end the use of "gay panic" and "trans panic" defenses in federal court — a defense that argues that the revelation of an individual's sexual orientation or gender identity helped provoke a defendant's violent reaction.

Markey is already facing two lesser-known candidates: Shannon Liss-Riordan, a workers' rights lawyer, and Steve Pemberton, a former senior executive at Walgreens.

The contest could be expensive.

Markey reported having more than $4 million in his campaign account as of June 30. Kennedy reported having slightly more — $4.2 million — in his House campaign account as of the same period.