Tammy Murphy, the first lady of New Jersey, gained crucial support in her bid for the U.S. Senate on Monday night, winning the Democratic convention vote in Bergen County by a decisive margin.
It was Ms. Murphy’s first victory at a convention decided by delegates using secret ballots, and it was considered a must-win matchup in her primary battle against Representative Andy Kim, a third-term South Jersey Democrat.
The vote was 738 for Ms. Murphy, 419 for Mr. Kim.
Over the past month, Mr. Kim, 41, won the first five county conventions in New Jersey that permitted delegates to select a nominee, including in Monmouth County, where Ms. Murphy and her husband, Gov. Philip D. Murphy, have lived for 25 years.
With the support of Mr. Murphy, who has nearly two years left in his term, Ms. Murphy, 58, racked up early endorsements from a raft of influential Democratic officials even before hitting the campaign trail.
But she has been battered by claims of nepotism and has struggled to gain support among rank-and-file Democrats as she makes her case that she should be their nominee to run for a coveted seat now held by Senator Robert Menendez, a Democrat who has been charged with taking bribes. Mr. Menendez has not ruled out running for re-election, but he has been abandoned by most Democratic leaders and has almost no path to victory.
Bergen County, in northern New Jersey, is the state’s most populous region. It has more Democrats than any county other than Essex, which includes Newark, the state’s largest city.
“Things are going as I had hoped,” Ms. Murphy told reporters after she was declared the winner on Monday. “Obviously, it’s great to have this incredible win.”
Starting at 5 p.m., delegates began slipping paper ballots into locked silver-and-red boxes on tables that ringed a union hall in Paramus, N.J. The room remained filled for most of the evening, as people milled about chatting with friends while waiting for the votes to begin being tallied by hand at 7:30 p.m. Many people wore embroidered patches or clothing suggesting that they were members of labor unions or worked for local public safety departments.
Supporters of both Ms. Murphy and Mr. Kim said that they had no memory of a convention that had drawn more interest from the news media — or the candidates.
“Calls, mail, texts,” said Rob Rohrberger, 64, of Glen Rock, describing the barrage of political messages sent by candidates.
Ms. Murphy helped lead Mr. Murphy’s two campaigns for governor, but she has never before run for office. She was a registered Republican until 2014, when Mr. Murphy was gearing up to run for governor.
Ms. Murphy, the mother of four grown children, worked for several years as a financial analyst after graduating from college, but she has not worked outside the home since marrying Mr. Murphy 30 years ago. She has since sat on an array of philanthropic boards and has been a champion of improving maternal and infant mortality rates as first lady.
Bergen County’s Democratic chairman, Paul A. Juliano, a former public works director in Fairview, was appointed by Mr. Murphy last March to a $280,000-a-year state job as chief executive of the New Jersey Sports and Exposition Authority. He endorsed Ms. Murphy soon after she entered the race in November, and he has been actively campaigning for her.
Elizabeth Heap, a longtime member of the Democratic Committee of Bergen County, said that she cast a ballot for Ms. Murphy.
“It’s about time New Jersey had a woman in the Senate,” Ms. Heap, 74, said as she left the union hall.
Mr. Rohrberger said that he had received personal calls from both candidates, but that he had decided to vote for Mr. Kim, a former national security adviser who counseled generals during the war in Afghanistan. Mr. Kim flipped a seat held by a Republican in 2018 to be elected to Congress, and he has been re-elected twice.
To Mr. Rohrberger, it was Mr. Kim’s experience that made the difference.
Ms. Murphy has also been selected as the nominee in Passaic and Union Counties. Those decisions, however, were made by a handful of party leaders in closed-door meetings.
Last week, Mr. Kim filed a federal lawsuit that took direct aim at the backroom party politics that undergird the state’s longstanding practice of permitting county leaders — be they Democrat or Republican — to place preferred candidates for all races in a single row or column on ballots. It is known as “the line,” and it is unique to New Jersey. Unendorsed candidates appear off to the side, in a nearby row or at the ballot’s edge, a location commonly referred to as “ballot Siberia.”
A recent study by a Rutgers University professor showed that placement on the so-called line afforded candidates for Congress a 38-percentage-point advantage.
Mr. Kim has asked a federal judge, Zahid N. Quraishi, to overturn the practice and to instead require candidates to be grouped together according to the office they are seeking. Judge Quraishi is expected to hold a hearing on the case on March 18, a week before the deadline for candidates to file nominating petitions.
Ms. Murphy has lagged behind Mr. Kim in several surveys, including the first independent poll of the race, and last month her campaign manager stepped down, suggesting a possible shift in strategy.
She has since released her first television ad, filmed in front of the National Rifle Association’s headquarters in Virginia, and has announced an endorsement by Howard Dean, the former governor of Vermont. She held a fund-raiser last week in Massachusetts with the state’s governor, Maura Healey; on Wednesday, Gov. Kathy Hochul of New York is expected to host a fund-raiser for Ms. Murphy with suggested donations of $3,300 to $6,600.