U.S. Republican presidential front-runner Donald Trump came under fire from his rivals on Thursday for saying Muslims hate the United States at a debate where the gut-punching attacks of earlier forums gave way to a suddenly civil tone with a serious focus on the issues.

Trump, who has voiced skepticism about U.S. military involvement abroad in the past, for the first time said America's effort against ISIS militants might require between 20,000 and 30,000 U.S. troops, a number similar to what some Republican hawks have proposed.

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The CNN-hosted debate at the University of Miami was crucial, coming days before votes in Florida and Ohio that will determine whether U.S. Sen. Marco Rubio of Florida and Ohio Gov. John Kasich will be able to continue with their increasingly long-shot candidacies.

With previous assaults on Trump having failed to knock him down, Rubio and U.S. Sen. Ted Cruz of Texas chose a more civil approach, raising questions about Trump's policy positions without attacking him personally.

Trump, for his part, used the debate to try to attract establishment Republicans, saying he is generating support from non-Republicans who could help carry the party to victory in the Nov. 8 election. And he eschewed the inflammatory, personal attacks on his rivals that have drawn both cheers and boos in prior debates.

"The Republican Party has a great chance to embrace millions of people that it's never known before. They are coming by the millions. We should seize that opportunity," he said.

But he stuck to positions that many establishment Republicans reject, such as his belief, as stated in television interviews, that followers of Islam "hate us."

"We have a serious problem of hate. There is tremendous hate," said Trump, who has proposed a temporary ban on Muslims entering the United States.

Rubio, Cruz and Kasich said the United States needs to maintain good relations with Muslim countries in the Middle East to help in the fight against ISIS militants.

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"We are going to have to work with people in the Muslim faith even as Islam faces a serious crisis within it," Rubio said.

Rubio also defended American Muslims as patriots.

"If you go anywhere in the world you're going see American men and women serving us in uniform that are Muslims," he said.

"Anyone out there that has the uniform of the United States on and is willing to die for this country is someone that loves America," he added.

Rubio shifted to a more positive tone after his anti-Trump tirades of the past two weeks. But he and Cruz repeatedly sought to raise questions about Trump's policy positions from trade to the Middle East.

Cruz pointed to areas where Trump has been a late-comer to the conservative movement, such as his past support for Democratic causes and candidates. He also noted how Trump has asked his supporters at rallies to demonstrate support by raising their right hand, a scene that produced photographs that some critics said looked like Nazi Germany.

"At Donald‘s rallies recently he’s taken to asking people in the crowd to pledge their support to him. I have to say I think that's exactly backwards. We are here pledging our support to you, not the other way around," Cruz said.

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Trump, in discussing how he would consider placing between 20,000 to 30,000 U.S. troops on the ground to defeat Islamic State militants, vowed to complete the mission quickly and bring troops home to focus on rebuilding the United States.

“We really have no choice, we have to knock out ISIS,” Trump said. “I would listen to the generals, but I’m hearing numbers of 20,000 to 30,000.”