Republican presidential candidate Donald Trump addresses the Sunshine Summit in Orlando, Fla., on Friday. (John Raoux/AP)

Immigration has emerged as the dominant issue of this Republican primary campaign. It helped make billionaire Donald Trump the front-runner, after his calls for a massive wall on the U.S.-Mexico border set him apart from the field.

The attacks on Paris Friday brought to the fore the question of how the U.S. should deal with the influx of refugees from Syria. That capped off a week where immigration had already been a hot-button issue, sparking a fight between two men who think they will be the front-runner, if Trump fever ever subsides. Sen. Ted Cruz (R-Texas) attacked Sen. Marco Rubio (R-Fla.) for opposing measures to improve border security. In response, Rubio attacked Cruz for . . . agreeing with Rubio, that some undocumented residents should be given a path to legal status. Cruz now says that he didn’t actually mean that when he proposed it: It was part of a complicated parliamentary plan to make Democrats look bad.

The Washington Post took a close look at the immigration proposals of the five top-polling GOP candidates: Trump, retired neurosurgeon Ben Carson, Rubio, Cruz and former Florida governor Jeb Bush.

In broad terms, the examination showed a fundamental breakdown in the field. On one side were Trump and Carson: political novices, trying to build an immigration platform from the ground up. Their ideas often sound simple, but would include new and complicated measures to carry out.

Sen. Marco Rubio (R-Fla.) and Sen. Ted Cruz (R-Texas) appear on Capitol Hill in 2013. (J. Scott Applewhite/AP)

Trump’s plan calls for a wall on the border, and a “deportation force” to carry out an unprecedented, expensive roundup of undocumented residents at home. Carson’s plans call for a new category of “guest workers,” and for military-style drone strikes on smugglers’ hideouts (though not if the smugglers are inside).

On the other side were Rubio, Cruz and Bush. All are political veterans who actually have a record on immigration — but are now trying to back away from parts of it. Rubio and Cruz, especially, have tried to avoid a difficult question that they had already — and publicly — proposed measures to solve.

That often-dodged question? How to deal with the undocumented immigrants who live in the U.S. already.

Question #1: How have the Paris attacks played into the immigration debate?

All five candidates have spoken out since the Paris attacks to address the issue of immigration.

Trump has already said that he would eject all Syrian refugees from the country, if elected. On Saturday, he reinforced that position, saying the Paris attacks highlighted how much of a risk countries take when accepting those immigrants.

“What is going on is terrible,” Trump said. “And when you look at what happened in that case: It was just reported, one from Syria and our president wants to take in 250,000 from Syria.” (The Obama Administration has actually committed to helping only 10,000.) Trump continued: “And we all have heart, and we all want people taken care of and all of that, but with the problems our country has, to take in 250,000 people — some of whom are going to have problems, big problems — is just insane. You have to be insane. Terrible.”

Bush and Cruz said that the United States should still accept refugees — as long as they are Christians. “We should focus our efforts as it relates to refugees on the Christians that are being slaughtered,” Bush told CNN’s “State of the Union” in an interview on Sunday morning. Cruz, meanwhile, said on “Fox and Friends” that the country could continue to be a “safe haven” for Christians but not “refugees that may have been infiltrated by ISIS.” He went on to say Muslim refugees should be resettled in majority Muslim countries.

Rubio said on “This Week” Sunday that the U.S. should not take any refugees from Syria, because it is impossible to vet people trying to enter the country. “It’s not that we don’t want to,” he said. “It’s that we can’t, because there’s no way to background check someone who is coming from Syria.”

Carson also said that accepting any refugees was out of the question. “To bring them over here is ... a suspension of intellect,” he said on “Fox News Sunday.”

Question #2: What more should be done to stop illegal immigration?

All five of the GOP candidates say they support increasing security at the U.S.-Mexico border, by increasing manpower and electronic measures.

Trump, of course, goes further. He wants to build a wall.

He has also said that he could make the Mexican government pay for the wall, with a series of squeeze tactics. Trump would seek to impound all the “remittance” money that illegal immigrants send home to Mexico, and increase fees on border crossing and visas. “We will not be taken advantage of anymore,” Trump’s immigration plan proclaims.

Cruz, speaking about immigration on Friday, handed out a policy proposal saying he would “build a wall that works.” Cruz didn’t immediately provide details about the kind of wall he wanted to build.

Carson has said he wants to build an asphalt road along the border, with fences on both sides, and station one Border Patrol agent every quarter mile. Also, like several other candidates, Carson has called for using more drone surveillance over the border — despite the disappointing results that surveillance drones have yielded so far. But Carson went beyond that, and suggested actually using weaponized drones that could use airstrikes to destroy “caves” used by human smugglers.

“There are caves that they utilize. Those caves can be eliminated,” Carson told CNN in August. “I’m not talking about killing people. No. People. With. Drones.” Carson said.

But it wasn’t clear how he would be certain the caves were empty before the drones eliminated them.

Away from the border, in the interior of the U.S., all five candidates have called for broadly similar changes. They would all like to give employers more ability to check the immigration status of new hires, to better track those who overstay their visas, and to crack down on “sanctuary cities,” where local authorities do not enforce federal immigration laws.

Question #3: Should undocumented residents be deported en masse?

Trump, of course, says yes — they should be deported by force, if necessary. Trump didn’t come into the campaign with this plan. As recently as July, in fact, Trump seemed open to letting some illegal immigrants stay. “If somebody’s been outstanding, we try and work something out,” he told one interviewer.

Now, Trump has a very different plan.

He wants to model his deportation on an Eisenhower-era program, which was much smaller and ended brutally for some immigrants: they died of heatstroke, by the dozens, when U.S. authorities left them in desert towns in Mexico. Trump’s plan would require an enormous amount of money and law-enforcement manpower — and could be bogged down in the country’s already-overloaded immigration courts.

“You’re going to have a deportation force,” Trump said this week.

Carson, Rubio and Bush have rejected the idea of a Trump-style mass deportation.

Cruz has refused to answer the question, saying that the fate of these 11 million should not be decided before the U.S. border is secure.

Question #4: If undocumented residents are not deported, what should happen to them?

Carson would give current undocumented immigrants six months to register with the U.S. government, pay “a back-tax penalty,” and pass a background check. If approved, they would be allowed to live and work legally in the U.S. as “guest workers.”

“If they don’t register within that six-month period, they’re criminals,” Carson said this week. “And are treated as such.”

Carson has not been clear about whether these guest workers could ever be allowed to obtain U.S. citizenship. “In terms of them becoming citizens later on down the road, if they have done things the right way, we, the American people, will decide what the criteria for that would be,” said in September.

Bush has made his empathy for undocumented immigrants a hallmark of his career; he once famously called illegal immigration “an act of love” for one’s family.

But now, even Bush has pulled back on the generosity of his plans for this group.

Bush, for instance, once called for giving all undocumented residents a chance to become citizens. But now, he says he wouldn’t. Instead, he would offer the majority of them a chance to become legal residents — if they learned English, paid back taxes and fines and passed a background check.

Now, Bush’s plan is to offer only a limited number of undocumented residents a chance at citizenship. Only those brought here as children could apply, and only if they had gone to college or joined the U.S. military.

As for Rubio, it’s clear what he did support, in the past. The present is fuzzier.

In 2013, Rubio co-sponsored a massive immigration-reform bill that offered illegal immigrants a path to legal status, and then — after 13 years — a way to gain full citizenship. The plan required immigrants to pay fines, pass a background check and learn English.

Now, however, Rubio has retreated to a more tentative position. He says he doesn’t want to deport illegal immigrants en masse. That seems to indicate he’d like to give many of them legal status. But he doesn’t want to talk about how that would work — or whether it would also give them a path to citizenship — until the border is secured first.

“I think the mood of the majority of Americans is we understand we need to do something reasonable and realistic for people that have been here for a significant period of time and are other law-aiding, but we’re not going to do anything until we ensure that this never happens again,” Rubio said in a press conference in Florida Friday. “And that is why it is so important that it begin on the front-end by proving that we’ve made progress on reducing the number of people that are coming into the country illegally.”

Cruz is even more opaque about what he wants.

The Texas senator has railed against “amnesty.” But, in 2013, Cruz actually laid out a plan to give undocumented immigrants legal status, in an amendment he proposed in the Senate. Like Carson’s plan, it would have created a class of guest workers, who were allowed to live and work in the U.S., but not given a means to obtain citizenship.

Now, Cruz says that wasn’t a real plan. It was a Senate trolling move, dressed up as legislation. He’d proposed the amendment solely to make Democrats take a difficult vote.

“Those amendments were introduced in an effort to defeat the Gang of Eight amnesty bill,” Cruz said in an interview with the Post’s Dan Balz. “They were introduced in an effort to demonstrate the hypocrisy of the Democrats who were supporting this.” Specifically, Cruz wanted to demonstrate that Democrats were dead-set on giving immigrants a path to citizenship, so they could benefit from those new citizens’ votes later on.

In this election, Cruz hasn’t proposed a detailed solution for the 11 million, saying that the debate must wait until the border is secure.

Question #5: Should the children of illegal immigrants become U.S. citizens at birth?

Trump, Carson and Cruz have all criticized this practice, called “birthright citizenship,” which is rooted in the 14th Amendment to the Constitution.

“I think birthright citizenship as a policy matter doesn’t make sense,” Cruz said in an interview in August on CBS’ “Face the Nation.” “We have right now upwards of 12 million people living here illegally. It doesn’t make any sense that our law automatically grants citizenship to their children because what it does is it incentivizes additional illegal immigration.”

Carson’s campaign said he is open to a more aggressive tactic: trying to prevent pregnant women from entering the U.S. at all, if they are “seeking to enter the U.S. simply to give birth,” a spokesman wrote.

Carson’s plans create a new problem for him to solve: what would become of the children of the “guest workers” his immigration plan would create? A spokesman said Carson was leaning against granting those children citizenship, either.

Bush and Rubio have said they do not want to end birthright citizenship.

Question #6: Should President Obama’s executive actions on immigration be cancelled?

Obama has issued two executive orders intended to shield millions of illegal immigrants from deportation. One, known as Deferred Action for Childhood Arrivals (DACA), has allowed hundreds of thousands of people — all brought to the U.S. as children — to remain in the U.S. and obtain work permits. The other, called Deferred Action for Parents of Americans and Lawful Permanent Residents (DAPA), would allow those undocumented residents to apply for work permits. It would affect up to 5 million people, but it has been halted by the courts.

All five top GOP candidates have said they would end these orders.

Rubio, however, has left open the idea of extending the DACA program — shielding those who arrived as children — for a time. It might remain in place, he says, while he pressed Congress for a broader immigration form that might shield the same group by other means.

But Rubio has said that DACA must end, even if Congress doesn’t act.

Question #7: Do the rules governing legal immigration need to be changed?

Trump has called for a “pause” in the issuance of all new “green cards,” which give an immigrant legal, permanent-resident status. “Before any new green cards are issued to foreign workers abroad, there will be a pause where employers will have to hire from the domestic pool of unemployed immigrant and native workers,” Trump’s immigration plan says.

None of the other four have called for that kind of pause. Cruz, however, proposed Friday to make sure the level of overall legal immigration does not rise, while “American unemployment remains unacceptably high.” The U.S. unemployment rate is now 5 percent, which is the lowest level in more than seven years.

The candidates have also disagreed about whether to increase the number of H-1B visas, which allow high-skilled foreigners to work in the U.S. for a limited period. The visas are especially beloved in high-tech fields, but they’ve also been criticized by people who believe they displace Americans from the same jobs and lower prevailing wages.

Cruz had been a longtime proponent of expanding the number of these visas, which are now capped at 65,000 per year. But in recent weeks, Cruz has changed his mind, stating that he is concerned about abuses of the H-1B system. And on Friday, he went the opposite direction, saying he would order that no H-1B visas be issued for 180 days, while the government investigates those abuses.

In the past, Rubio and Bush have also called for increasing the cap. Trump has called for altering the H-1B program so that it gives American workers a better chance to compete for the same jobs.

Dan Balz, Jose A. DelReal, Jenna Johnson, Jerry Markon, Ed O’Keefe and Katie Zezima contributed to this report.