SIMI VALLEY, Calif. — Carly Fiorina emerged Wednesday as a formidable challenger to front-runner Donald Trump, as issues such as national security got a serious airing in the second Republican debate and suggested a turning point in the 2016 campaign.

Trump will likely remain the front-runner, and his quips, style and insults will be the most buzzed about features of this second presidential debate. But most of the encounter at the Ronald Reagan Presidential Library showcased policy differences, sometimes stark, sometimes nuanced, and saw Fiorina scrapping to elbow Trump out of the spotlight.

The debate’s tone signaled that the campaign is moving into a more serious phase. As election days get nearer, people begin to judge candidates as potential presidents, demanding less entertainment and more depth and gravitas.

Trump still commanded more attention than anyone. CNN, one of the debate’s sponsors, covered his arrival at the Ronald Reagan Presidential Library. Questions about Trump dominated the early debate featuring the four second-tier contenders. The opening minutes of the main event, historically the most watched part, featured Trump blasting away at his rivals and challengers swinging right back.

Trump continues to tap voter frustration with government and politics as usual. But it became clear that Fiorina is poised to move up fast.

She had a big opening, having shined in the August debate among lower-polling candidates. Then Trump criticized Fiorina’s appearance, allowing her to get feisty fast. “Women heard very clearly what Mr. Trump said,” Fiorina said, with a stern, knowing look that seemed to last an eternity. The audience cheered.

Trump came back with a half-smile. “She’s got a beautiful face and I think she’s a beautiful woman,” he said, that smile evaporating into a look of exasperation.

Fiorina stood out because of more than a quip. The former corporate executive illustrated in staccato-like style a command of foreign affairs, rattling off details about world trouble spots, talking in personal terms about what it would take to curb threats from Russian leader Vladimir Putin.

“Having met Vladimir Putin, I wouldn’t talk to him at all,” she said.

She engaged with Trump in an exchange over how each ran their companies. With a look of controlled anger, she tersely noted how his companies filed for bankruptcy four times. He countered, raising his voice at one point, how he has earned billions of dollars.

And she got personal. After rivals sparred over marijuana policy, Fiorina added a somber note. “My husband, Frank, and I buried a child to drug addiction,” she said.

Lori Ann Fiorina, one of her two stepdaughters, died in 2009. She was 35, and had struggled with drug abuse and alcoholism.

She may get a challenge from Ben Carson, who has been edging close to Trump in recent polls. Carson acted as though the real estate mogul wasn’t there. In his calm, almost somnambulant style, the retired neurosurgeon said he “won’t get into describing who’s a politician and who’s not a politician.”

He wouldn’t engage in an immigration fight with Trump. Carson has questioned Trump’s plan to deport undocumented immigrants, but he said Wednesday he’d be willing to listen. Voters, particularly in Iowa, have shown they like Carson’s understated ways, but those don’t play well in debates.

Others who badly need to revive their teetering campaigns weren’t so reluctant, and they didn’t help themselves.

Scott Walker, the governor of Wisconsin, tried the aggressive approach.

Trump fired back, noting that Walker’s claims of successfully managing his state’s budget were overblown. Wisconsin has had balanced budgets since Walker became governor in 2011, which the law requires, but lawmakers have struggled to erase big deficits.

Lost in this crowd were some big Republican names, notablyNew Jersey Gov. Chris Christie and former Arkansas Gov. Mike Huckabee.

Among those who did speak up, Jeb Bush and Rand Paul had the most to lose and probably didn’t gain much Wednesday. Both entered the 2016 race with big expectations, Bush because of his political network and fundraising ability and Paul because he inherited from his father a devoted legion of libertarians.

Bush charged that Trump tried to influence him to support a Florida casino and insisted Trump apologize for upsetting his wife.

Trump praised Bush’s wife but wouldn’t apologize. He was dismissive about the casino. He insisted if he had wanted the casino, he’d have gotten it.

Marco Rubio was often forgotten in the early going, too, but his campaign’s strategy has been to methodically build an organization and start moving when it matters most.

Chances are the debate won’t dethrone Trump, but it illustrated why he’s got to get serious. He needs to show he’s more than a vehicle for voter outrage. If not, others are ready for center stage.