(CNN) Cory Booker was, before this week, an afterthought in the crowded 2020 Democratic race, with half a dozen other candidates outpacing him in the polls and eclipsing him in the national conversation.

But a direct confrontation with Democratic front-runner Joe Biden over the former vice president's comments about working with segregationist senators has placed him suddenly and squarely in the spotlight.

It is not a moment Booker expected or planned.

After Biden this week touted his ability to work with prominent segregationist senators in the 1970s, and quipped about the pejorative term "boy," Booker stepped in and spoke out on Wednesday, calling on Biden to apologize in a sharp rebuke of the former vice president.

And that might have been that. Until Biden called on Booker to apologize, and Booker said he wouldn't, and the two men found themselves on a call that was ultimately leaked Thursday to the press.

Now, after a quiet start to his campaign, Booker will head to South Carolina for a high-profile weekend of campaigning, followed by the first debates next week, as a candidate in the national conversation.

"It sets the stage for Booker to show he is a serious candidate," said Brady Quirk-Garvan, a former chair of the Charleston, South Carolina, Democratic Party who has endorsed Booker.

The decision to directly engage Biden marked a stark departure for the New Jersey Democrat, who has sought to run a relentlessly positive campaign and, as a result, been reluctant to criticize rivals by name. It was also a political gamble, with Biden extremely popular among Democratic voters and leading the 2020 field.

But speaking out against Biden "was not a political calculation," a campaign aide told CNN. "We were not thinking, 'This is going to break us out of the pack.'" Nor was it the sign of a broader strategic shift underway, they said.

Instead, Booker described it to one aide as a "moral moment," saying, "Even if I was not running, this would still be something I would speak out about."

As Booker explained on CNN on Wednesday, Biden's comments resonated with him on a personal level, recalling the "indignity" of "watch(ing) another man call my father 'boy'" or being called that himself.

"These are the kind of things that do cause hurt and harm," Booker added.

Brian Fallon, a Democratic strategist who was Hillary Clinton's press secretary during the 2016 campaign, said Booker needed to thread a delicate needle in targeting Biden while maintaining a moral high ground, "but they executed it flawlessly."

"Even though Booker was criticizing Biden, it was very hard to argue with the point that Booker was making, and so it didn't come off as opportunistic," said Fallon. "It wasn't a drive-by hit. It was thoughtful, and it was something that he had unique moral standing on."

"It looks obvious now that he should have done it," Fallon added. "But at the time, I'm sure it was a it was a decision that some courage."

As outrage built Wednesday morning over Biden's comments, however, it wasn't initially clear whether Booker or any of Biden's other rivals would speak up.

Then Booker retweeted a photo of a downcast girl with the caption, "me realizing i'm going to have to vote for joe biden in 2020." He added his own short, uncharacteristically sassy retort: "No, you won't."

As the post began racking up likes, retweets, and emoji eyes, Booker's aides were surprised to see it pop up on their Twitter feeds. Unbeknownst to them, the senator had tweeted the response himself, a campaign aide told CNN, as he fumed over Biden's comments.

It was the first hint of what would become a defining moment in Booker's campaign thus far. Roughly an hour later, Booker released his statement sharply rebuking Biden and calling on the former Vice President to apologize, setting off a cascade of similar reactions from other presidential candidates.

A few hours later, the move also drew a reaction from Biden, who said he saw no need to apologize. "Cory should apologize," Biden added, stunning Booker's campaign and prolonging the controversy further.

But Booker, in an interview Wednesday evening with CNN's Don Lemon, did not back down. "I was raised to speak truth to power," Booker said, "and I will never apologize for doing that."

After the interview wrapped, the cycle culminated with the call from Biden to Booker, which was described as "respectful" by sources on both sides, but in which no apologies were exchanged.

In the call, Booker "shared directly what he said publicly, including helping Vice President Biden understand why the word 'boy' is painful to so many," said Sabrina Singh, a press secretary for Booker's campaign. "Cory believes that Vice President Biden should take responsibility for what he said and apologize to those who were hurt."

In an interview Thursday evening on MSNBC, Booker said the call lasted roughly 15 minutes. For Booker, the appearance marked his second night in a row commanding a 30-minute block on primetime cable news.

The episode could have played out very differently. A first draft of Booker's statement rebuking Biden did not call upon the former vice president to respond, the Booker campaign aide said. But Booker and his aides agreed that it was important and added it.

For Booker's campaign staff, Booker's decision to speak out marked an affirming moment in a tough slog that so far has offered little payoff.

"Wednesday night for so many folks was, 'This is why we're in it,'" said the campaign aide.

It could also mark a new phase of the campaign for Booker.

"If yesterday was a sign that they're gonna be willing to mix it up a little bit and be smart about doing so," said Fallon, "then I could see that being the beginning of a turning point."

For now, however, this moment in the spotlight appears poised to continue for Booker.

Booker and Biden will both be in South Carolina this weekend, along with most of the other Democratic candidates, as they attend Rep. Jim Clyburn's famous fish fry and the state Democratic Party convention.

At the latter event, by a random stroke of fortune, Booker and Biden will speak last, back-to-back.