The Republican presidential contenders toned down their disputes at Thursday's debate, discussing in a civil manner the issues of trade, education, and Social Security, rather than offering the rollicking clashes that defined their forum in Michigan last week.
The cautious opening rounds underscored the increasingly high stakes facing each of the four remaining candidates on stage, as well as the damage incurred by the loudest combatants during the previous event.
The 12th Republican candidates' debate sets the stage for judgment day next Tuesday for Florida Sen. Marco Rubio and Ohio Gov. John Kasich in their respective home states. Both need to win contests on their home turf to stay in the race.
Front-runner Donald Trump has been telegraphing for weeks that he was seeking to take on a more presidential posture and be a unifying figure in the party. And Texas Sen. Ted Cruz had little incentive to rock the boat as he emerges as the chief alternative to Mr. Trump by virtue of his primary wins rather than by confronting the New York businessman.
In a rare dust-up, Mr. Rubio took exception to Mr. Trump's recent comments about Muslims hating Americans. Using the evocative image of the Islamic star and crescent on U.S. military graves, Mr. Rubio said: "Presidents can't just say anything they want," Mr. Rubio said. "It has consequences, here and around the world."
The Florida senator then seized on comments Mr. Trump made defending his remarks, saying he was trying not to be "so politically correct."
"I'm not interested in being politically correct," Mr. Rubio shot back. "I'm interested in being correct."
For the most part, though, the candidates pulled their punches and steered the conversation away from direct confrontations, even when the debate moderators put them on the spot. The contest was a dramatic shift from the 11 previous debates that were defined by vitriolic arguments between the crowd of candidates. "So far, I can't believe how civil it's been up here," Mr. Trump remarked at one point.
He opened the night by urging his Republican doubters to take a step back and view his success--and the waves of new voters he is drawing into the system--as a boon for the party. He cited the "millions and millions" of people shattering turnout records as evidence he is expanding the party, attracting Democrats, independents and Republicans who don't often vote.
"Frankly, the Republican establishment, or whatever you want to call it, should embrace what's happening," Mr. Trump said.
The GOP candidates sidestepped their divisions, including the long-term structure of Social Security. When Mr. Trump was questioned about his refusal to call for a revision of the popular retirement program for seniors, he turned his ire on the Democrats rather than draw a contrast with his rivals who support broad changes to ensure its financial integrity for the future.
Even Mr. Rubio, who is desperate for a game-changing moment to rejuvenate his flagging campaign, dramatically shifted tone after two straight debates hammering Mr. Trump for his inconsistent statements on key issues, business practices that undermine his hard-line position on immigration and his personal behavior.
Mr. Trump hurled some barbs at Mr. Cruz, challenging his stance on ethanol subsidies and accusing him of supporting "amnesty" for immigrants in the U.S. illegally--a charge the Texas senator has denied and bait he refused to take in the Miami debate.
Instead, Mr. Cruz shot back by reminding viewers that Mr. Trump has been a longtime donor to Democrats as well as Republicans in what he called the Washington establishment.
"It's very hard to imagine how suddenly this candidate is going to take on Washington," Mr. Cruz said.
Mr. Trump also tussled--gently--with Mr. Rubio over how to make Social Security solvent. Mr. Trump, aligning himself with the Democratic candidates, said he would do "everything in my power" not to change the system and promised instead to "make this country rich again so we can afford it." Mr. Rubio, who has proposed gradually raising the retirement age, called out Mr. Trump's policy, saying, "The numbers don't add up."
Mr. Cruz doesn't face the same must-win crucible Tuesday as Messrs. Kasich and Rubio, but if Mr. Trump wins the two biggest battlegrounds of Florida and Ohio, the Texas senator will find himself in a deep delegate hole, with time running out to close the gap.
As he closes in on the nomination, questions are mounting about what kind of standard-bearer Mr. Trump would be for the Republican Party. Mr. Trump, who has been registered as a Democrat and an independent, said his views were supported by "millions of people pouring into the polls" and that he differed with his party on only one issue: trade.
"The jobs in this country are disappearing, especially the good jobs," he said. "Trade deals are absolutely killing our country."
Mr. Cruz said Mr. Trump is "right about the problems but his solutions don't work." He accused him of shifting positions frequently, saying "I don't know where he'll be tonight."
Mr. Trump responded by accusing Mr. Cruz of shifting positions on the renewable fuel standard. The Texas senator refused to respond to that attack as well, although during the Iowa caucuses he noted that he has always opposed the program as corporate welfare but has adjusted how quickly he'd end it.
The more civil tone might prove a welcome break for candidates, but it was unlikely to deliver the kind of breakout performance that Mr. Rubio, in particular, needs to get his campaign back on track heading into a must-win primary in the Sunshine state.
Polls in Florida heading into Thursday's debate showed Mr. Trump with a comfortable lead. Mr. Kasich, in contrast, is running close to Mr. Trump in his home state of Ohio.
Mr. Trump, asked whether his businesses' frequent hiring of foreign workers clashed with his campaign rhetoric on immigration, said he takes advantage of laws but would change them as president. "We're allowed to do it," he said. "Nobody knows the system better than me. Nobody else on this stage knows how to change it."
The candidates largely avoided clashes on immigration reform. Mr. Kasich, whose father was an immigrant, said he would be "running for president of Croatia" if the U.S. didn't allow immigration. Mr. Trump said foreign-worker employment practices that his own businesses use are "unfair for our workers," and called for pausing foreign-worker visas for at least a year.
Mr. Cruz accused Democrats of supporting illegal immigration "because they view those immigrants as potential voters" and said the U.S. is currently bringing in "far too many low-skilled workers." And Mr. Rubio said a key criteria for incoming immigrants should be "what skills do you have, what business are you going to open."
Write to Beth Reinhard at firstname.lastname@example.org and Rebecca Ballhaus at Rebecca.Ballhaus@wsj.com