His plan to deport the estimated 11 million people living illegally in the US was decried as impractical during an election debate in Wisconsin.
But he said on Wednesday it was cheap, "doable" and humane.
Democratic President Barack Obama wants to lift the threat of deportation to millions of undocumented people.
But his plan to allow about four million people to apply for work permits has been hindered by legal challenges.
Mr Trump, a billionaire New Yorker who has been leading in the polls, defended his plan to build a wall on the US-Mexico border and deport all the people living illegally in the US.
"You're going to have a deportation force, and you're going to do it humanely," he told MSNBC's Morning Joe.
"You have millions of people that are waiting in line to come into this country and they're waiting to come in legally."
How do you deport 11m people?
Debate exposes Republican divisions
He pointed to the actions of former US President Dwight Eisenhower, who authorised what was probably the largest deportation operations in US history.
Senator Ted Cruz backed Mr Trump during the debate, condemning any form of "amnesty" because illegal immigration kept wages low for American workers.
But his fellow Republican, Ohio Governor John Kasich, said that mass deportation was a "silly argument" that could never work and would split families.
The issue has exposed the diverging views within the party, and Florida Senator Marco Rubio clarified his stance on Wednesday.
He described his position as a "very long path" to citizenship when people had spent at least 10 years on a work permit.

The musician penned the haunting track, He Was Alone, after spending time at a refugee camp in southern Turkey on the Turkish/Syrian border.
"We see this human disaster and we see numbers in millions and we're trying to make sense of it," he told the BBC.
"We've forgotten how to understand the human story behind this."
Three-year-old Syrian boy Kurdi made headlines around the world in September when he washed up on a Turkish beach.
"What I came to realise is that it's the reductionism which has caused much of the problem," said Yusuf. "That we've forgotten how to understand the human story behind this and so it's that young soul that I visualised in my head when I wrote this this song."
He said that, when the shocking pictures of Kurdi appeared in the press and on TV in 2015, "suddenly people woke up a bit for a while and said, 'Oh God, that looks like my little nephew or my little son', and so that came to my mind."
But he also lamented any initial public galvanisation over the humanitarian crisis in the West, saying it had long since dissipated.
"It's kind of, 'Well that's news today and tomorrow it's old - and people move on and politics gets involved. It is quite sad that this happens and it is the politics which I think interferes with it."
Proceeds from the new record will go toward the #YouAreNotAlone campaign and Save The Children.

Alfie Hyett was found by his mother Louise Hembrow at home in Hereford last March.
Police told the inquest they found no evidence to support a claim he was copying a choking game that encouraged people to block their airways.
A verdict of accidental death was recorded at Hereford Town Hall.
More on this and other stories from Hereford and Worcester
When the inquest opened in June, Mark Bricknell, senior coroner for Herefordshire, heard Alfie died of vagal inhibition, which causes the heart to stop through the stimulation of the vagus nerve in the neck.
Before the inquest, Ms Hembrow told the BBC she believed her son had taken part in the game where the brain is starved of oxygen to create an adrenalin rush.
However, Det Sgt Mark Jinks, from West Mercia Police, said there was no evidence he had researched the game or discussed it with friends.
Mr Bricknell said he believed Alfie had tied the scarf around his neck out of childhood curiosity or experimentation without realising the potential consequences, and had not meant to take his own life.

Ex-servicemen and women joined many members of the general public to reflect on those who died or have been injured in service to their country.
Here are images from some of the acts of remembrance from those who took part.
Donald Trump has defended his hardline stance on immigration, a day after it was attacked by fellow Republican presidential candidates on national TV
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Singer-songwriter Yusuf, formerly known as Cat Stevens, has said the harrowing death of Syrian toddler Aylan Kurdi inspired his latest song.
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A 10-year-old boy found hanged in his bedroom "did not understand the risks of putting a scarf around his neck", a coroner has said.
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The nation fell silent for two minutes on Wednesday to remember the nation's war dead on Armistice Day.