Donald Trump has gone from reality television star to frontrunner for the Republican presidential nomination because of one issue – immigration.

His campaign has been defined by incendiary anti-immigration rhetoric and it has found support across America with Trump securing 46% of the delegates that have been on offer so far.

The billionaire has publicly targeted Mexicans and Muslims; and while today might be a different world, the GOP stance on immigration has not always been as extreme.

36 years ago during a primary debate between then-Republican presidential candidates Ronald Reagan and George H.W. Bush in Houston, both championed immigrants living in the United States when faced with the question: “Do you think the children of illegal aliens should be allowed to attend Texas public schools for free, or do you think their parents should pay for their education?”

Bush passionately replies to the young Texan saying: "If those people are here, I would reluctantly say they would get whatever it is, what society is giving their neighbours. But the problem has to be solved ... We're creating a whole society of really honourable, decent, family-loving people that are in violation of the law, and secondly we're exacerbating relations with Mexico.”

“The answer to your question is much more fundamental than whether they attend Houston schools, it seems to me. I don't want to see ... six- and eight-year-old kids, being made, you know, totally uneducated, and being made to feel that they're living outside the law.”

“Let's address ourselves to the fundamentals. These are good people, strong people. Part of my family is a Mexican."

Similar to Bush, Reagan favoured a pragmatic approach.

"I think the time has come that the United States and our neighbours—particularly our neighbour to the south—should have a better understanding and a better relationship than we've ever had.”

"Rather than making them, of talking about putting up a fence, why don't we work out some recognition of our mutual problems, make it possible for them to come here legally with a work permit, and then, while they're working and earning here, they pay taxes here.

“And when they want to go back they can go back, and cross. And open the border both ways, by understanding their problems.”

Donald Trump: What are his actual policies?

If Trump does become President, there would be a lot more than “talk about putting up a fence”.

One of his landmark election pledges is to “build a great, great wall” on America’s southern border costing at least $8 billion. His Republican rivals Ted Cruz and Marco Rubio have also proposed similar plans.

Border controls are not a new phenomenon, there has been greater calls ever since the 9/11 terrorist attacks, but the language has dramatically changed.

“Honourable, decent, family-loving people” have now become “people that have lots of problems” and according to Trump, “they’re bringing drugs. They’re bringing crime. They’re rapists.”

In pictures: US Elections 2016 Show all 15 1 /15 In pictures: US Elections 2016 In pictures: US Elections 2016 Democratic U.S. presidential candidate Bernie Sanders speaks to supporters after rival candidate Hillary Clinton was projected as the winner in the Nevada Democratic caucuses Reuters In pictures: US Elections 2016 Democratic presidential candidate Hillary Clinton takes photos with workers at her campaign office in Des Moines, Iowa AP In pictures: US Elections 2016 Republican presidential candidate Mike Huckabee, second from left, prays before lunch with supporters at Drake Diner in Des Moines, Iowa Reuters In pictures: US Elections 2016 Democratic presidential candidate and former Maryland Governor. Martin O'Malley, speaks during a campaign stop in Waterloo, Iowa AP In pictures: US Elections 2016 Democratic presidential candidate Bernie Sanders speaks, as his wife Jane O’Meara Sanders looks on, at a campaign event at Iowa State University Getty In pictures: US Elections 2016 Republican presidential candidate Marco Rubio speaks at St. Ambrose University in Davenport, Iowa Reuters In pictures: US Elections 2016 Republican presidential candidate Ben Carson speaks at a campaign event at Fireside Pub and Steak House in Manchester, Iowa. Getty In pictures: US Elections 2016 Republican presidential candidate Rick Santorum visiting supporters at a house party in West Des Moines, Iowa Reuters In pictures: US Elections 2016 Republican presidential candidate Donald Trump at a campaign event in Cedar Rapids, Iowa AP In pictures: US Elections 2016 Republican candidate Ted Cruz campaigns at Greene County Community Centre in Jefferson, Iowa AP In pictures: US Elections 2016 Senator Rand Paul speaks during a Caucus rally at his Des Moines headquarters in Iowa Getty In pictures: US Elections 2016 Republican candidate Jeb Bush speaks at a campaign event in Des Moines, Iowa AFP In pictures: US Elections 2016 Former Alaska Governor Sarah Palin introducing the arrival of Donald Trump during a campaign rally in Cedar Rapids, Iowa Reuters In pictures: US Elections 2016 A portrait of Democratic candidate Bernie Sanders at his campaign headquarters in Des Moines, Iowa Getty In pictures: US Elections 2016 Campaign badges on sale ahead of a Trump rally at the Ramada Waterloo Hotel and Convention Centre in Waterloo, Iowa Getty

This harsh rhetoric has led to concern from both sides of the political spectrum.

Hillary Clinton, who is leading the Democratic presidential nomination, commented: "One of the most distressing aspects of this campaign has been the language of Republican candidates, particularly their front-runner, that insults, demeans, denigrates different people."

"He has cast a wide net. He started with Mexicans. He's currently on Muslims.”

Hector Barreto, head of the Small Business Administration in George W Bush’s White House, said last year: “The danger we’re facing right now is if this kind of rhetoric is not checked, if this becomes normal, if the other political candidates feel: ‘Maybe I need to talk more like Trump.’”