Joseph Kennedy III is in — a source close to the U.S. Representative saying the latest member of the storied political dynasty to serve in Congress will announce his primary challenge to elder statesmen U.S. Sen. Edward Markey on Saturday.

After weeks spent testing the waters, Kennedy will make his Senate run official at an East Boston Social Centers breakfast with supporters and the local community Saturday morning, the source said. He will then tour the state in the ensuing few days.

Kennedy’s entrance into the race could upend Massachusetts politics and divide the party. The split was already on display among activists at last weekend’s state Democratic convention, some of whom supported a fresh face in the seat while others said Kennedy, 38, should wait his turn and back down from challenging Markey, 73, whom was widely praised for his co-sponsoring of the Green New Deal.

“It’s game on,” Democratic strategist Scott Ferson said.

Kennedy and Markey spoke Wednesday, according to Markey’s campaign, which released a statement saying, “Elections are about choices, and Ed looks forward to spending the next 14 months campaigning hard every day.”

The campaign said Markey will focus on climate change, income inequality, gun reform, universal health care, reproductive freedom and immigrant rights in his re-election bid.

Kennedy will announce while riding a wave of favorable polling that puts him significantly ahead of his veteran opponent as he seeks to reclaim his family’s Senate mantle.

“Kennedy name recognition is going to beat a guy named Markey just any day,” Ferson said. “But people will clearly know Ed Markey and what he’s done by the time we get to next September.”

Markey, who’s spent four decades in Congress, has been staffing up and pushing out endorsements from U.S. Sen. Elizabeth Warren and U.S. Rep. Alexandria Ocasio Cortez (D-NY), his Green New Deal co-sponsor. He put on a show of force at the state Democratic convention, with campaign volunteers rushing the stage — as instructed in an email obtained by the Herald — as he delivered a rousing keynote emphasizing his working-class roots.

Kennedy acknowledged at a post-convention meet-and-greet his challenge could “put some folks in an awkward spot” — including Warren, for whom he’s done surrogate work.

But he told reporters, “I don’t think primaries are something people should shy away from.”

Two others are already in the race — business executive Steve Pemberton, and labor attorney Shannon Liss-Riordan, who is in Kennedy’s district.

Liss-Riordan indicated she wasn’t backing down Wednesday.

“Change won’t happen if we keep sending the same politicians to Washington,” she said in a statement. “I’m offering a fresh perspective and a new voice and have been energized by how our campaign has been received. I knew getting into this that it would be a difficult battle, but I’ve taken on difficult battles my entire career and don’t plan on stopping now.”

Pemberton said, “I have been running for Senate not to be the candidate of the privileged or the insiders, but to represent the rest of us who too often feel left out and forgotten by those who have been in Washington — I know that experience well. l want to bring a sense of urgency to the Senate that currently lacks lived experience and too often defaults to incumbency and insider connections.”

Kennedy’s Senate bid also opens up the 4th Congressional District, for which Ihssane Lecky has already announced.