Nobody Saw Andy Kim Coming. That’s What He Was Counting On.

Facing federal charges that he accepted bribes, including cash, gold bars and a Mercedes-Benz, Senator Robert Menendez announced on Friday, Sept. 22, that he would not resign.

A day later, Andy Kim, a little-known Democratic congressman from southern New Jersey, gathered his top advisers for a conference call. Everyone present assumed that Mr. Kim would announce his intention to challenge Mr. Menendez for his Senate seat.

The question was when.

Zack Carroll, who was Mr. Kim’s campaign manager during his first race for Congress in 2018, told the group that a typical campaign launch takes six weeks. After all, he would need to hire staff, compile a list of potential donors and create a website, a rollout video, a media plan and some slogans. And Mr. Kim would need the blessing of political bosses across the state, an especially urgent task in New Jersey, where local party leaders have held enormous sway over primary elections for generations.

“You don’t upset a two-term incumbent by flying by the seat of your pants,” Mr. Carroll said.

Mr. Kim listened quietly. Then he read aloud his campaign announcement.

“What if I were to announce in three hours?” Mr. Kim said.

The announcement, which Mr. Kim posted on social media that afternoon, kicked off perhaps the luckiest Senate campaign in modern New Jersey history. Over the next six months, Mr. Kim went from underdog to front-runner, outmaneuvering Tammy Murphy, the wife of Gov. Philip D. Murphy, who joined the race in November and quickly won the support of New Jersey’s powerful Democratic Party machine.

In late March, Mr. Menendez said he would not run in the party’s primary. Three days later, Ms. Murphy ended her campaign.

Mr. Kim is not yet a U.S. senator. There are two other candidates in the Democratic primary and four candidates running on the Republican side, although no Republican has won a Senate seat from New Jersey since 1972. Mr. Menendez has also left open the possibility of running for re-election as an independent. But Mr. Kim has now become the odds-on favorite.

Along the way, he also won a stunning ruling in federal court in March barring party chairs from designing ballots in this June’s Democratic primary that give preferential treatment to their endorsed candidates, potentially upending New Jersey politics into the future.

“It is probably the most significant shift in New Jersey politics in decades,” said Steven Fulop, the mayor of Jersey City and a candidate for governor in 2025.

Over his six years in Congress, Mr. Kim, 41, has appeared to his constituents as a nerdy and earnest public servant, said Patrick Murray, director of The Polling Institute at Monmouth University in New Jersey. This image went national on Jan. 7, 2021, when he was photographed gathering trash left by rioters in the U.S. Capitol rotunda.

“I thought he was a staffer,” said Andrew Harnick, who took the pictures for The Associated Press. “I’m not sure I’ve ever seen something like that before, a member of Congress on his hands and knees, picking up trash.”

This year’s Senate race has showcased two other traits that have proved central to Mr. Kim’s political career: An insider with the tactics of an outsider, he often seizes big moments that others miss.

“He’s like Clark Kent,” said Chris Russell, a Republican strategist who managed two failed congressional campaigns against Mr. Kim. “He comes off as inoffensive and well meaning and nice. Behind the scenes, this guy is a political animal. He is very shrewd, very calculating.”

Mr. Kim grew up in Moorestown, N.J., the same suburb of Philadelphia where he now lives. After high school he attended Deep Springs College, an elite two-year institution in an otherwise deserted valley in California. At the time, enrollment at the school was limited to 26 students.

News of the Sept. 11 terror attacks left the college’s students and 10 staff and faculty members feeling paralyzed, said Jack Newell, then the college’s president. Mr. Kim, then 19 years old, asked to organize a gathering that evening to process the attacks.

“That proved to be a turning event for all of us,” Mr. Newell said. “Responding to an emergency situation is an instinct. And Andy is one of the people who does have it.”

Mr. Kim later graduated from the University of Chicago and won a Rhodes Scholarship to study at Oxford University. Next came a speedy rise in foreign policy, including time advising Gen. David Petraeus, then the commander of American forces in Afghanistan. At age 31, Mr. Kim became the only member of the National Security Council advising President Barack Obama on Iraq.

It was a sleepy assignment until 2014, when the terrorist group ISIS led a military campaign across northwestern Iraq aimed in part at exterminating the Yazidi, a Kurdish religious minority. Tens of thousands of Yazidis fled into the Sinjar mountains, where they were trapped without water or food.

Mr. Kim’s job in the White House seemed mundane: schedule interagency meetings and draft agendas. But he managed to get leaders focused on saving the Yazidis, even under a president who had built his political career on ending America’s military engagement in Iraq, said Tom Malinowski, then an assistant Secretary of State.

On the morning of Aug. 7, 2014, Mr. Kim chased down Denis McDonough, the president’s chief of staff. Vaulting over layers of protocol, Mr. Kim, the junior staffer, described the urgent situation to Mr. McDonough, who asked him to draft possible responses.

That evening, the president authorized a plan, drafted by Mr. Kim, to airdrop supplies to the Yazidis, followed by airstrikes on ISIS militants intent on genocide.

“He is an extremely deliberate, thoughtful, careful guy,” Susan Rice, the national security adviser at the time, said of Mr. Kim. “But also gutsy.”

After Donald J. Trump was elected president in 2016, Mr. Kim moved back to New Jersey to challenge Tom MacArthur, a two-term Republican congressman, former insurance executive and millionaire who paid most of his own campaign expenses. Camden County’s powerful Democratic Party machine had decided that the seat was impossible for a Democrat to win, Mr. Kim said in a recent interview, so he did not seek its support. He did enlist his influential friends, however, including Ms. Rice, who organized a fund-raiser.

“‘I don’t frankly think you’ve got a great shot’,” Ms. Rice said she told Mr. Kim at the time. “‘But I’ll do what I can.’”

Mr. MacArthur had sponsored legislation to let insurance companies charge higher prices to people with pre-existing conditions. Mr. Kim made it the central issue of his campaign, describing his opponent as indifferent to his constituents’ lives.

Republican campaign aides say Mr. Kim’s strategy hit below the belt. Mr. MacArthur’s mother died of cancer when he was 4; years later, his 11-year-old daughter died from a rare neurological disorder.

“He tries to portray the nice-guy image, but he proved himself to be rather nasty,” said Adam Geller, who ran polling for Mr. MacArthur.

Mr. Kim won by less than 4,000 votes. A month after he was sworn in, he introduced legislation to cut prescription drug prices for seniors; a version of the bill later passed with the 2022 Inflation Reduction Act. He also worked to maintain funding for a large military base in his district, and held monthly town halls.

After he spent two terms representing an area where Mr. Trump won twice, redistricting turned Mr. Kim’s district safely Democratic. He cast that safety aside to challenge Mr. Menendez — without even a phone call to Democratic Party bosses — a move many insiders believed would end his career.

“If he were a careerist, he wouldn’t have announced” so quickly, said Mr. Murray at Monmouth University. Without boss support, “you were probably a dead duck,” he said.

Others viewed the move as cunning.

“There was one lane for someone to run as the principled outsider against the machine that Menendez had represented,” said Mr. Malinowski, the former assistant Secretary of State, who won a New Jersey race for Congress in 2018 and had considered challenging Mr. Menendez himself. “And there would be a great advantage to whoever occupied that lane first.”

The first lady’s challenge

Tammy Murphy entered the race in November. A political neophyte, Ms. Murphy won immediate support from county bosses including in Bergen County, home to one of the largest blocks of registered Democrats in the state. The county’s party chair, Paul Juliano, is also president and chief executive of the New Jersey Sports and Exposition Authority, a job that pays $280,000 a year and was awarded by Ms. Murphy’s husband, the governor.

“I didn’t come from a powerful, politically connected family,” said Mr. Kim, whose father was a geneticist and mother was a nurse. “The first lady, this is her first time running, and she’s coming into this as part of the most powerful political family in New Jersey.”

But Democratic voters, particularly progressive activists, expressed frustration with what they considered nepotism, and her campaign struggled to catch fire.

“Tammy Murphy represents the arrogance of the party bosses,” said Valerie Huttle, a former New Jersey state assemblywoman who was ostracized from the party after she challenged a boss-endorsed candidate for state senate in 2021. “That’s what I think helped Andy.”

If his decision to challenge Mr. Menendez without asking the party’s permission was an insult to New Jersey’s Democratic Party machine, Mr. Kim’s subsequent lawsuit seeking to end the machine’s most powerful tool — control over election ballot design — was a direct attack.

Mr. Kim sued the state’s county clerks in federal court, arguing that the ballots designed by county bosses violated his constitutional rights. Candidates endorsed by their local party appear together in the same row. Those who fail to win the endorsement are relegated to “ballot Siberia,” Mr. Kim argued, making it difficult for voters to find them.

Grassroots activists, who had spent years fighting for reform, were ecstatic.

“Andy was the right person at the right time to take on the establishment,” said Yael Niv, president of the Good Government Coalition of New Jersey.

Mr. Kim’s campaign has focused more broadly on government reform, betting that voters will view structural issues like gerrymandering and the filibuster as blocking progress on abortion rights, gun control and health care.

“Process and policy, you can’t separate the two,” he said in an interview. “In order for me to pass women’s reproductive rights in America, we have to have filibuster reform.”

Mr. Kim also has pursued an inside game. His campaign raised $2.7 million by January, enough to be “armed to the teeth” with television ads in the New York and Philadelphia television markets, he said. He also attended up to five house parties a day, an organizing effort aimed at winning the same county nominations he was attacking in court.

“We talked a lot about how he could run both the inside and outside campaign,” Mr. Malinowski said of conversations with Mr. Kim in late 2023. “The bosses would feel the heat.”

Five days after Ms. Murphy dropped out, a federal judge granted Mr. Kim’s request for an emergency injunction, ordering a new ballot design for the Democratic primary.

The decision capped a remarkable six months in New Jersey politics in which two of the state’s most powerful politicians fell like dominoes, Democratic Party bosses lost the linchpin of their control over elections, and Mr. Kim — through a combination of tactical skill and sheer luck — became both the beneficiary and face of a once-in-generations change in New Jersey politics.

“This race is not only going to produce a new senator, but a completely new way of picking our leaders in New Jersey,” Mr. Malinowski said. “No one would have predicted it a year ago.”