At the CNN debate in Miami, GOP candidates sparred over immigration, social security, how to talk about Muslims and more. Here are the key moments. (Sarah Parnass/The Washington Post)

Through 12 Republican debates, there has been one consistent dynamic: Donald Trump has held center stage, literally and figuratively. He is the alpha politician who has fended off multiple opponents with cutting insults, timely interruptions and only an occasional exploration of the substance of policy.

Trump shared a debate stage Thursday night with his three remaining rivals, Sen. Ted Cruz of Texas, Ohio Gov. John Kasich and Sen. Marco Rubio of Florida, and he found a different way to control the evening: by deflection and adaptability.

Ahead in the race for the nomination, he adopted a more restrained and subdued demeanor, even passing up opportunities to strike back when his opponents tried to engage him. It was a strategy common to front-runners — play not to lose, avoid mistakes or eruptions, and force the opposition to change the dynamic.

For much of the evening, the four candidates carried on a generally civil discussion on the issues. They avoided the kinds of clashes that had created a downward spiral in their dialogue over the three previous debates.

Thursday’s encounter, in particular, seemed a direct reaction to the universal criticism of their debate a week ago, a forum that took the GOP campaign into the gutter. But in the more subdued environment, Trump was challenged anew to move beyond generalities, and he still struggled to explain where he really stands on a range of issues, from education and trade policy to Social Security and the federal budget deficit to dealing with the Islamic State and Iran.

That Trump has certain skills as a candidate is without question. He can dominate a debate or a news cycle with relative ease. His ability to keep opponents at bay and off balance has been stellar. But there is much more to being president than that, which is why there are so many doubts about him among the electorate at large. What the debates have shown is that Trump’s lack of depth on issues continues to be a key part of the story of his quest for the presidency.

Trump arrived at Thursday’s debate at the University of Miami nearing what could be a key turning point in the Republican campaign. By Tuesday night, after a round of primaries in big states, he either will be seen in full command of the nomination process — virtually unstoppable — or facing competition that could carry on all the way to the floor of the GOP convention in Cleveland in July with no certain outcome.

The New York billionaire has gotten to this moment through the systematic destruction of his opponents. He is a ruthless attacker and a pitiless counterpuncher. He reads people and goes for the jugular. He delights in putting down his opponents and shows no mercy as he does so.

Three candidates in particular have threatened him, and he has lashed at each. He unnerved Jeb Bush by declaring the former Florida governor the “low energy” candidate. When he called Rubio “Little Marco,” the tag seemed almost instantly to diminish the politician in whom so many establishment Republicans had invested their hopes.

Playing Trump’s game has proved foolhardy. Bush vacillated in his response to Trump’s insults until his candidacy had little oxygen left. Rubio finally responded when his candidacy was in deep trouble. Goaded into a direct battle, Rubio went down to Trump’s level of insults. He said this week he now regrets what he did. Rubio’s faint hopes of continuing depend on winning his home state on Tuesday, but he trails in the polls.

“Lyin’ Ted” is Trump’s belittling shorthand for Cruz. The two seemed to share a bromance for months, until it was clear to each that the other represented a mortal threat to his hopes of becoming the nominee. Cruz, however, has shown more resilience in the face of Trump’s attacks than Bush or Rubio, at least so far. He is second in states and delegates won.

Kasich has avoided attacking Trump and, in turn, has avoided being attacked. But can anyone beat Trump by trying to ignore him, as Kasich has done? Not likely. If the governor beats Trump in Ohio on Tuesday, he’s likely to find himself under attack, and a new chapter in their relationship will begin to unfold.

Washington Post-Univision Poll: Florida primary voters

But if Trump’s rivals have failed to knock him down by playing his game of insult and attack, they also have struggled to capitalize on those limitations, which says something about both them and Trump. If the New York businessman ultimately is denied the nomination — or eventually the presidency, should he become the nominee — it could be as much his fault as anything else. After nine months as a politician, there is still a question as to how much he has grown as a candidate.

On Thursday, he was pressed to explain his opposition to the education policy known as Common Core. “Education through Washington, D.C.,” he replied. When CNN’s Jake Tapper pointed out that Common Core was developed by governors and the states and that the standards are voluntarily adopted by states and localities, Trump said, “It has been taken over by Washington.”

When Kasich was asked about Common Core, he offered a deeper description of what he has done in Ohio on a range of education issues. Cruz, who opposes Common Core, outlined a series of proposals to expand charter schools, home schools, school vouchers and the like. “It’s easy to talk about the problem, but you have to understand the solutions.”

CNN’s Dana Bash pressed Trump to explain how he would preserve the financial viability of Social Security without making adjustments in the retirement age, benefits or anything else. Trump said he would attack waste, fraud and abuse. When Bash told him that estimates suggest that would accomplish very little, Trump shifted his focus to the cost of U.S. military obligations overseas.

This time it was Rubio who suggested the front-runner has limited knowledge of the problems he wants to solve. “The numbers don’t add up,” he said of Trump’s proposal. “The bottom line is, we can’t just continue to tiptoe around this and throw out things like I’m going to get at fraud and abuse.”

Trump responded by claiming that changes in the government’s bidding process would reap major dividends. “We’re going to go out to bid in virtually every different facet of our government,” he said. “We’re going to save a fortune.”

So it went through much of the debate — the most substantive discussion of any of the debates on an evening when Trump’s rivals sought to undermine him on the substance rather than theatrics.

On foreign policy, Trump was told that he didn’t understand that his Middle East policies were anti-Israel, even though he claimed that he was more pro-Israel than anyone else on the stage. He was accused of wanting to continue policies closer to those of President Obama and former secretary of state Hillary Clinton rather than change direction, as many Republicans are demanding.

Cruz delivered a cutting description of Trump’s foreign policy philosophy as, “China bad, Muslims bad.”

It was an entirely different approach by Trump’s opponents, and response by the New York billionaire. The criticisms produced few fireworks and probably muted headlines on Web pages all over the country.

Trump continued on a plane of generalities more than specifics. That has worked for him through the course of the nomination battle, but it leaves unanswered exactly what he would do as president.