U.S. Rep. Joe Kennedy III will challenge U.S. Sen. Ed Markey for his seat, setting up a showdown between one of the longest-serving Massachusetts lawmakers and a popular member of one of the nation’s most well-known political families more than three decades Markey’s junior.

A source close to Kennedy, 38, confirmed to WBUR that he will launch his campaign on Saturday in East Boston, where the Newton Democrat's great-great-great grandparents settled when they immigrated from Ireland in 1849.

Kennedy’s challenge to Markey forces Democrats in Massachusetts and beyond to pick sides between two well-liked and respected party members. The potential of the primary battle has stirred concern among some Democrats who fear the move will steer much-needed energy away from efforts to beat President Trump and House and Senate Republicans in 2020.

Markey and Kennedy spoke Wednesday. In a statement, Markey’s senior campaign adviser John Walsh said Markey earned voters’ backing in 2013, and plans to do it again.

"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 said. "From lowering drug prices for our seniors, to expanding opioid treatment and recovery services, to creating green jobs for our workers, Ed Markey is ready to continue the fight for Massachusetts."

Aides said Markey, who's 73, plans to campaign on his more than four-decade record in Congress, fighting for clean air and energy policies.

Markey has gathered a host of endorsements for his reelection in recent weeks, including from U.S. Rep. Alexandria Ocasio-Cortez, the lead House co-sponsor of Markey’s Green New Deal bill.

The New York progressive congresswoman — who at 29 is the youngest member the House — helps Markey blunt Kennedy’s pitch that it's time for generational change in the Senate. It also helps Markey to position himself as the more progressive choice over Kennedy.

In a campaign video, Ocasio-Cortez called Markey “a proud and strong progressive champion” on climate change.

“When I first got to Congress and we started to discuss big, bold plans — a solution on the scale of the crisis — many members shied away,” Ocasio-Cortez said.

Other high-profile endorsements for Markey include Massachusetts Sen. Elizabeth Warren and Sen. Catherine Cortez Masto of Nevada, who chairs the Democratic Senatorial Campaign Committee. Markey also has the backing of a majority of the state’s congressional delegation. A spokeswoman for Markey told WBUR more endorsements would be announced this week.

But polls suggest Kennedy has broad support from voters. A Suffolk University/Boston Globe poll earlier this month shows Kennedy with 9 percentage point lead over Markey in a five-person race, with 35% of likely Bay State voters choosing Kennedy and 26% Markey. In a head-to-head match-up, Kennedy bests Markey 42% to 28%.

Kennedy confirmed he was mulling a Senate bid three weeks ago. Since then, he's been talking to people in Massachusetts, gauging support for a run.

And he's spoken in favor of a few policy ideas, including getting rid of the legislative filibuster, lifetime tenure for Supreme Court justices and the Electoral College. So that's a clue he's going to be running on a message of structural change.

As of the end of June, both Markey and Kennedy had a little more than $4 million in their campaign accounts.

Two other candidates — Shannon Liss-Riordan and Steve Pemberton — have announced Democratic challenges to Markey as well.

Markey was elected to the seat in a 2013 special election after John Kerry was nominated to be secretary of state. Markey won election to a full term in 2014.

Kennedy has served in the House since 2013.

Kennedy’s challenge also sets up a potential competitive race for his 4th District congressional seat, which stretches from Brookline down to Fall River. Democratic state Treasurer Deb Goldberg has said she'd consider running for the seat.

With reporting by WBUR's Benjamin Swasey