Absent Donald Trump, the Republican presidential candidates strained to take advantage of a rare opportunity to step out of the front-runner's shadow in Thursday night's debate — a staid, policy-heavy contest that offered a glimpse of what the contest might have been without the unpredictable businessman.

Still, the candidates couldn't resist mocking Trump for boycotting the final debate before Iowa kicks off voting in the 2016 campaign on Monday.

Texas Senator Ted Cruz, who is locked in a tight contest with Trump in Iowa, opened the debate with a sarcastic impression of the real-estate mogul's frequent insults of his opponents.

"I'm a maniac and everyone on this stage is stupid, fat and ugly," Cruz said. Then he thanked his fellow candidates for showing Iowa voters respect by showing up.

Former Florida governor Jeb Bush, a frequent target of Trump, said with a wry smile, "I kind of miss Donald Trump; he was a teddy bear to me."

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U.S. Republican presidential candidate Donald Trump speaks at a veteran's rally in Des Moines, Iowa, on Thursday instead of participating in a Republican debate on Fox News. (Rick Wilking/Reuters)

Angry over an escalating feud with debate host Fox News, Trump bowed out of the debate and held what his team called a "Special Event to Benefit Veterans Organizations" at a packed 775-seat auditorium at nearby Drake University instead.

"When you're treated badly, you have to stick up for your rights," Trump said in explaining he was skipping the debate because he felt Fox News had dealt with him unfairly. Broadening his point, he said, "We have to stick up for ourselves as people and we have to stick up for our country if we're being mistreated."

Cruz, Rubio spar over immigration 

Trump's absence put the spotlight on Cruz, who is challenging for the lead in Iowa, as well as on Florida Senator Marco Rubio, who needs a strong showing in the state to stay competitive.

The two senators engaged in a lengthy debate on immigration, one of the most contentious issues among Republicans. Both have been accused of shifting their stances on legalizing some of those in the United States illegally, a position opposed by many GOP voters.

GOP 2016 Debate

Ted Cruz took jabs at the absent front-runner Donald Trump early into the Republican debate. (Chris Carlson/Associated Press)

Cruz accused Rubio of making a "politically advantageous" decision to support a 2013 Senate bill that included a pathway to citizenship, while the Florida senator said his rival was "willing to say or do anything to get votes."

In a rare standout debate moment for Bush, the former Florida governor sharply sided with Cruz in accusing Rubio of having "cut and run" on the Senate immigration bill.

"He cut and ran because it wasn't popular with conservatives," Bush said.

With their White House hopes on the line, the candidates worked hard to cast themselves as best prepared to be commander in chief and take on terror threats emanating both from abroad and within the United States.

Ben Carson said he's proud to be the only Republican presidential candidate without political experience.

GOP 2016 Debate

Marco Rubio Rubio defended his call for monitoring mosques in the U.S. in light of recent attacks by Islamic extremists. (Chris Carlson/Associated Press)

Carson, a retired neurosurgeon, says even though he hasn't made political choices, he's made plenty of life-and-death decisions during his career as a doctor.

"I don't think you need to be a politician to tell the truth," he said during Thursday night's Republican debate.

Candidates tout plans to fight ISIS, terrorism 

Carson said the key is to take new approaches to solve problems that have left many Americans terrified.

Rubio struck an aggressive posture, pledging that as president he would go after terrorists "wherever they are. And if we capture them alive, we're sending them to Guantanamo."

Rubio also stood by his previous calls for shutting down mosques in the U.S if there were indications that the Muslim religious centres were being used to radicalize terrorists.

GOP 2016 Debate

Retired neurosurgeon Ben Carson says he's proud to be the only Republican presidential candidate without political experience. (Chris Carlson/Associated Press)

Kentucky Senator Rand Paul — back on the main debate stage after being downgraded to an undercard event because of low poll numbers earlier this month — warned against closing down mosques. A proponent of a more isolationist foreign policy, Paul also raised concerns about the U.S. getting involved militarily in Syria, where the Islamic State group has a stronghold.

Taking aim at Clinton 

The candidates largely sidestepped direct confrontations with each other, focusing some of their most pointed attacks on Democratic front-runner Hillary Clinton.

"She is not qualified to be president of the United States," New Jersey Gov. Chris Christie said. "What we need is someone on that stage who has been tested who has been through it."

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Chris Christie argues a point during a Republican presidential primary debate on Thursday. (Chris Carlson/Associated Press)

Christie is part of a crowded field of more mainstream candidates who have struggled to break through in an election year where Trump, and increasingly Cruz, have tapped into voter anger with the political system.

Party leaders have grown increasingly antsy for some of the more traditional candidates to step aside after the first contests to allow one to rise up and challenge for the nomination.

'It's called politics'

Asked whether the crowded establishment lane was putting Trump in position to be the nominee, Bush said, "We're just starting out. The first vote hasn't been counted. Why don't we let the process work?"

Bush also defended the flurry of critical advertisements his well-funded super PAC has launched against Rubio and other rivals.

GOP 2016 Debate

Ohio Gov. John Kasich, right, answers a question as former Florida Gov. Jeb Bush listens during the Republican debate on Thursday. (Charlie Neibergall/Associated Press)

"It's called politics," Bush said. "That's the way it is. I'm running hard."

Trump pulled out of the debate this week, citing unfair treatment from host Fox News. He's feuded with the network for months, particularly anchor and debate moderator Megyn Kelly.

It's unclear whether Trump's unusual move will hurt his standing with Iowa voters. But his absence did give his rivals more time to make their case to voters.

With files from CBC News and Reuters