The second debate for top Republican presidential candidates included bashing Donald Trump, a fiery Carly Fiorina and an admission from Jeb Bush that he says his mom won't like. (CNN)

Something unusual happened here Wednesday when the Republican presidential candidates met for their second debate: For the first time since he joined the race, Donald Trump wasn’t the commanding presence on the stage.

Not that Trump wasn’t the Trump whom Americans have seen nonstop on cable television. Among the first words out of his mouth was a personal and unprovoked attack on Kentucky Sen. Rand Paul. He sparred at times angrily with Carly Fiorina over who was the better business executive. He and Jeb Bush, standing next to each other, had repeated exchanges.

But at other times, particularly when the discussion shifted from what Trump has said about the others to issues of domestic and foreign policy, the candidate who has dominated the summer and leads the polls was far less a force.

Unlike the debate in Cleveland last month, the other candidates arrived with no illusions about Trump’s candidacy — they take him seriously now — or the need for them to step up and show their own mettle, both in challenging Trump and in displaying their own attributes, records and character.

Over three hours of lively, entertaining and at times angry debate, Trump was put on the defensive as much as he tried to stay on the offensive. Whether that will change the course of the nomination battle won’t be known for some weeks. After the last debate, despite missteps, Trump rose rather than fell. But Wednesday showed that his rivals are ready to engage him, when necessary, both from long distance and to his face.

Republican presidential candidate Sen. Rand Paul said he's "very concerned" about having Donald Trump in charge of nuclear weapons. (CNN)

This was billed in advance as the debate that would highlight the current state of the Republican race, one in which the outsiders — Trump, Ben Carson and Fiorina — enjoy more than 50 percent of support against the insiders — the eight current or former elected politicians. Instead, it became a classic of debates past — poke the front-runner. It was Trump against the field, or rather the field against Trump.

The ebb and flow of the debate, guided by CNN’s Jake Tapper with the help of CNN’s Dana Bash and conservative radio talk show host Hugh Hewitt, oscillated between personal exchanges, many of them with Trump involved, and discussions of issues.

When immigration came to the fore, Trump was at the center of the debate, defending his hard-line stance that calls for deporting the millions of undocumented immigrants and challenging the 14th Amendment over the issue of birthright citizenship.

Bush fired back at him over those proposals. He invoked Ronald Reagan’s optimistic vision in contrast to what he said was Trump’s approach “that everything is coming to an end.” He said Trump’s proposals would cost hundreds of billions of dollars and tear families and communities apart. Trump disputed Bush’s comment that those who have come illegally came out of an act of love. “This is not an act of love. He’s weak on immigration. He doesn’t get my vote.”

Trump and Fiorina clashed memorably several times. Once was over their business records, and it was as pointed and sharp as any during the evening. He accused her of running Hewlett-Packard into the ground. She accused him of running up “mountains of debt” and filing for bankruptcy four times.

The other exchange, one everyone was waiting for, came when Fiorina was asked about Trump’s comment, captured in a Rolling Stone profile, denigrating her looks by saying, “Look at that face!”

Trump had earlier explained that he was talking about her “persona,” not her looks. Asked about that, Fiorina said acidly, “I think women all over this country heard very clearly what Mr. Trump said.” The audience responded with one of the biggest rounds of applause of the night. Trump’s response: “She’s got a beautiful face, and I think she’s a beautiful woman.”

See how candidates responded to Trump

Fiorina was the new addition to the main debate stage, after her performance in the undercard debate in Cleveland, and she came with the clear intention of making a memorable impression. She got another applause-meter moment with a ringing statement about Planned Parenthood and sought to project strength and confidence surrounded by 10 men in suits and ties.

At times, others stepped forward. Wisconsin Gov. Scott Walker, in danger after seeing his poll numbers plummet nationally and, critically, in Iowa, leapt into the conversation when the question on the table was whether Trump could be trusted with his hand on the nuclear codes. “We don’t need an apprentice in the White House,” Walker said. “We have one right now. . . . We don’t know who you are or where you’re going.”

One reason Trump seemed a less commanding presence was that on some issues, he offered little substance beyond reassurances that he would be strong and tough, a negotiator par excellence and someone who would grasp the complexities of national security issues as president — and would find experts to help him.

Challenged on how he would deal with the Russians putting military resources into Syria, he said he would know how to get along with Russian President Vladimir Putin, an answer that others seemed to find unsatisfying.

Florida Sen. Marco Rubio, who displayed his fluidity with foreign policy, challenged him implicitly on how much he knew about the world, saying he would be happy to have a longer discussion of the issues to see the depth of Trump’s knowledge.

Rubio wasn’t the only one who had a moment. Ohio Gov. John Kasich and Texas Sen. Ted Cruz sparred over what to do with the Iran nuclear deal. Paul expressed his support for diplomacy over war in dealing with Iran.

New Jersey Gov. Chris Christie upbraided Trump and Fiorina for talking about their business records and said the candidates should instead focus on the lives of middle-class Americans.

Carson, the retired neurosurgeon, displayed the same low-key, sober demeanor that has found a growing audience over the past six weeks. And near the end, he showed humor, when he tweaked Trump for having called him “an okay doctor.” After Trump had spoken about vaccinations, Tapper asked Carson for his view of Trump’s ideas. “He’s an okay doctor,” Carson said.

Former Arkansas governor Mike Huckabee defended his strong support for Kentucky county clerk Kim Davis for her resistance to giving marriage licenses to same-sex couples.

The prelude to Wednesday’s debate was strikingly different than the buildup before Cleveland. Then, many of the other candidates still were clinging to the belief — or hope — that Trump would quickly burn himself out, that his candidacy would prove a short-lived, if entertaining, sideshow.

By this month, that belief had disappeared. Trump’s continued strength atop the Republican field has forced all the others to rethink his potential and its impact on their hopes of winning the nomination. Some still believe he will sink under his own weight, and if they were not prepared to abandon their original strategies, they recognized that they had to make adjustments.

Last month’s debate in Cleveland marked the beginning of the end of a long exhibition season. Wednesday’s forum at the Ronald Reagan Presidential Library signaled a turn to the post-Labor Day phase of the campaign that, historically, brings more intensive campaigning, more debates, sharper engagement and heightened stakes for the candidates.

It was clear throughout the evening that everyone on the stage understood what’s now at stake. Trump may continue to dominate the polls, but if Wednesday’s debate was any indication, he can expect a bumpier ride in the weeks and months ahead.