BOSTON — Rep. Joe Kennedy III will challenge Sen. Ed Markey for his Senate seat in 2020, setting up a Democratic primary battle that has the potential to divide the Massachusetts political class and draw national attention.

Kennedy will formally announce his plans Saturday morning at a breakfast in East Boston, according to a source familiar with the news. In the days following the announcement, Kennedy plans to take a tour across the state, the source said. News of Kennedy’s announcement was first reported by the Boston Globe.

Kennedy met with Markey on Wednesday afternoon to inform him of the news, according to the source. Markey and Kennedy have steered clear of one another since news broke that Kennedy was considering a challenge for the seat. Over the weekend, Kennedy held a meet-and-greet with activists after the state Democratic party's annual convention to gather input on a Senate run.

The race between Markey and Kennedy is shaping up to be a contest colored by generational themes: Kennedy is 38 years old and Markey, 73, has been in Congress since the 1970s. Elected to the Senate in a 2013 special election, Markey has been rolling out endorsements from prominent lawmakers and environmental groups for weeks. Sen. Elizabeth Warren (D-Mass.) and Rep. Alexandria Ocasio-Cortez (D-N.Y.) are among his supporters. The Sierra Club endorsed Markey on Wednesday.

In late July, Kennedy quietly commissioned a poll testing his favorability against Markey, and campaign lines for both lawmakers. A public poll conducted just after Labor Day found Kennedy 14 percentage points ahead of Markey in a head-to-head match-up, and 9 percentage points ahead of Markey and two lesser-known primary challengers, executive Steve Pemberton and attorney Shannon Liss-Riordan.

“In 2013, Ed Markey asked voters to send him to the U.S. Senate to fight for the people of Massachusetts. Since then, he has fought on the front lines to show them they were right. 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 Markey campaign said in a statement.

“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. 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.”