Ms. Kennedy’s performance in her home base of Atlantic County was disappointing, compared with the strong support there for Joseph R. Biden Jr., the former vice president, in the presidential race. With about three-fourths of the expected votes in Atlantic County counted, Mr. Biden had a 9,000-vote edge on Mr. Trump. But Ms. Kennedy was leading Mr. Van Drew there by fewer than 4,800 votes.
“Jeff Van Drew has been around a long time, and he’s got cross-party support, but I was not expecting a drop-off like that,” said Michael Suleiman, chairman of the Atlantic County Democratic Committee. “I wish Amy had a better showing out of Atlantic County.”
Local observers attributed much of that falloff to the work of Craig Callaway, a notorious political operative whom Mr. Van Drew had paid more than $100,000 for his get-out-the-vote efforts.
Mr. Callaway, who served more than three years in prison for bribery, specializes in delivering mail ballots from low-income precincts in and around Atlantic City, a Democratic stronghold. State law was changed five years ago to limit to three the number of ballots a “messenger” like Mr. Calloway can deliver to a polling place or collection site. Mr. Callaway had worked for Ms. Kennedy during the Democratic primary, but after a falling out signed on with the Van Drew campaign.
“It’s been a controversial operation over time, but it’s never been actually proven vote fraud,” said John Froonjian, director of the William J. Hughes Center for Public Policy at Stockton University, in the state’s Galloway Township.
Mr. Callaway did not respond to messages seeking comment.
Ms. Kennedy, 42, carried only two of the eight counties — Atlantic and Cumberland — within the Second Congressional District, which stretches across the southern end of New Jersey from the Atlantic Ocean to the Delaware River. Mr. Van Drew overcame those deficits by winning in the more conservative counties of Cape May, Gloucester, Ocean and Salem. The district also includes small portions of Burlington and Camden Counties.
Mr. Van Drew was a vocal opponent of the president’s impeachment and joined the Republicans in December when it became clear that he could not count on re-election support from Democratic leaders in his South Jersey district, setting up a marquee House race infused with national angst.