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.