WASHINGTON — Representative Joseph P. Kennedy III of Massachusetts plans to announce on Saturday that he is challenging Senator Ed Markey for the Democratic Senate nomination in their state, setting up a titanic generational clash next year between an heir to the state’s foremost political dynasty and the longest-serving member of its congressional delegation.
Mr. Kennedy informed Mr. Markey of his plans today, according to a Democrat familiar with their discussion who spoke on the condition of anonymity to relate a private conversation. Mr. Kennedy, the grandson of Robert F. Kennedy, will announce his candidacy in a modest, East Boston neighborhood before embarking on a statewide tour.
The contest will almost certainly be the country’s most high-profile Senate primary and could offer clues for just how eager Trump-era Democrats are to replace more incumbent lawmakers. But unlike with some of the Tea Party-era Republican races between ideological foes, Mr. Kennedy, 38, and Mr. Markey, 73, are both reliable progressives and each is claiming support from liberal leaders and organizations.
Initial polling indicates that Mr. Kennedy would begin the contest as the front-runner. And some Massachusetts Democrats — uneasy about the prospect of a fractious, yearlong primary contest — had hoped that Mr. Markey would take a hint and bow out after word of Mr. Kennedy’s interest in the seat first leaked last month.
But Mr. Markey, who was first elected to the House in 1976, made clear again Wednesday that he intends to run for re-election.
“I’m going to run in all of the issues that I have been fighting for today and into the future: climate change, income inequality, a woman’s right to choose and on gun safety legislation,” Mr. Markey told reporters in the Capitol after The Boston Globe revealed Mr. Kennedy’s intentions, adding: “The response I have been getting across the state has been absolutely terrific.”
Mr. Markey’s senior campaign adviser John Walsh made no mention of Mr. Kennedy in a prepared statement, but did note that “elections are about choices.” In an interview last month, Mr. Markey emphasized his lengthy legislative record and blue-collar roots, previewing how he’d run against his younger, more affluent colleague.
Massachusetts has been riveted by the prospect of a Kennedy-versus-Markey showdown since August, after a report by The New York Times found that the political scion had begun telling senior Democratic officials that he was considering taking on Mr. Markey and had in fact commissioned a poll testing his prospects against the incumbent. Each Democrat has over $4 million in the bank for the campaign.
Some in Washington and Boston were skeptical that Mr. Kennedy would go through with the race, finding his calculation to be out of character for a young lawmaker who has sought to avoid controversy since being elected to the House in 2012. And Mr. Markey scrambled to lock down support, including from perhaps the most prominent Democratic primary challenger of 2018, Representative Alexandria Ocasio-Cortez, with whom he introduced the Green New Deal legislation.
But as Mr. Kennedy began traveling far from his southeastern Massachusetts district in recent weeks, it became clear he was serious about confronting Mr. Markey. And with a long roster of Massachusetts Democrats rising through the state’s political ranks, it became clear that running against Mr. Markey may prove to be his best chance to join the Senate where his grandfather and great-uncles, Edward M. and John F. Kennedy, once served.
A handful of Massachusetts Democrats began sounding notes of regret for getting behind Mr. Markey after it became clear Mr. Kennedy may run. But one leading figure in the state party said Wednesday she was remaining by the senator’s side.
“I endorsed Ed Markey last February,” said Senator Elizabeth Warren. “I couldn’t ask for a better partner in the United States Senate.”
But Ms. Warren, who taught Mr. Kennedy at Harvard Law School, noted that she has “known Joe since long before he got into politics.”
And asked if he was wise to pick this fight, she said: “I have no criticism.”