Representative Richard E. Neal of Massachusetts, one of the Democratic Party’s most powerful committee chairmen, beat back a challenge on Tuesday from a progressive insurgent in a race that embodied the clash between the party’s leadership in Washington and a younger generation of activists on the left.
Mr. Neal’s victory over Alex Morse, a 31-year-old mayor who had endorsements from a number of high-profile progressive politicians and groups, including Representative Alexandria Ocasio-Cortez of New York, broke a string of progressive triumphs that have recently upended senior House Democrats from New York to Missouri.
Late Tuesday, Mr. Neal was leading Mr. Morse by about 20 percentage points.
As chairman of the Ways and Means Committee, which has jurisdiction over the tax code and other legislation that raises revenue for the government, Mr. Neal was the most prominent target of progressive activists in Democratic primaries this year. But he was also one of the most prepared and best financed, with a war chest of nearly $4 million and a team of allied outside groups to help him fend off a flood of money and resources into the district in support of Mr. Morse.
Mr. Neal, 71, was endorsed by Nancy Pelosi, the speaker of the House, who has defended many Democratic incumbents against challenges from the left. Mr. Neal has been in Congress since 1989, the year Mr. Morse was born.
Unlike some other House Democrats who have lost primaries in recent years, Mr. Neal has fewer upscale liberals in his district, which covers a range of cities and small towns in Western Massachusetts.
While the race mostly turned on the policy differences between the two candidates — Mr. Morse, the mayor of Holyoke, campaigned on an agenda that included liberal environmental and health care policies that Mr. Neal criticized as largely unrealistic — events took a stunning turn last month when the student newspaper at the University of Massachusetts, Amherst, published anonymous complaints from some College Democrats who claimed Mr. Morse had acted inappropriately with students.
Mr. Morse, who is gay and taught as a visiting lecturer at the university until last year, acknowledged having consensual relationships with students over the years, none of whom were in his classes, he said. But his support on the left began to crumble, as progressive groups said they would stop campaigning for him and the university opened an investigation into his conduct.
But then the allegations started to unravel, leading Mr. Morse to accuse Mr. Neal and his allies in the Massachusetts Democratic Party of working behind the scenes on a homophobic smear campaign against him. Mr. Neal denied any involvement.
Reporting by The Intercept uncovered messages between some of the students showing that they had discussed how they might damage Mr. Morse’s campaign, with one student suggesting that it might help him get a job with Mr. Neal after graduation.
The state party acknowledged that it had provided minor legal advice to the College Democrats, who first made their allegations in a letter to Mr. Morse that cited vague complaints about how he made some students feel uncomfortable.
Once the messages surfaced, the groups and activists who had pulled their support for Mr. Morse got back in the race on his behalf. His campaign said it raised more than $410,000 in the weeks after the student paper published its report and received 800 queries from people hoping to volunteer.
But in the end, it was not enough to overcome the power of incumbency and clout that Mr. Neal had amassed in his 16 terms in the House.