Donald Trump - fighting for a clean sweep on Tuesday in primaries that could secure him the Republican nomination - likened immigrants to the US to a deadly snake that bites the person who shows it kindness.

Speaking in a private hangar at Youngstown airport, the Republican frontrunner underscored his staunch position on immigration, vowing to build one wall along the US border with Mexico to keep Central Americans out, and another one in Syria to keep Isis fighters in.

“We have to make sure we know who we’re letting in,” he said, to cheers.

Mr Trump was introduced by New Jersey Governor Chris Christie (AP)

Towards the end of a 35 minute speech delivered in front of around 1,500 people, Mr Trump recited the words to "The Snake", a 1968 song by Al Wilson.

The lyrics to the song, which Mr Trump has recited before, tell the story of an ill snake who is taken into her home by a kindly woman. The snake is nursed back to health, but then delivers a poisonous bite to its host.

The dying woman asks the snake why it has acted in such a way given that she has helped it. Mr Trump - delivering the snake’s response - concluded: “Oh shut up silly woman, you knew damn well I was a snake before you took me in.”

Mr Trump is looking for big wins in Illinois, Florida, North Carolina, Missouri and Ohio, all of which are holding primaries on Tuesday and two of which - Florida and Ohio - award their delegates on a winner-takes-all-basis.

He currently holds the lead in the first four of those states.

Donald Trump's most controversial quotes Show all 14 1 /14 Donald Trump's most controversial quotes Donald Trump's most controversial quotes On Isis: "Some of the candidates, they went in and didn’t know the air conditioner didn’t work and sweated like dogs, and they didn’t know the room was too big because they didn’t have anybody there. How are they going to beat ISIS?" Getty Donald Trump's most controversial quotes On immigration: "I will build a great wall — and nobody builds walls better than me, believe me —and I’ll build them very inexpensively. I will build a great, great wall on our southern border, and I will make Mexico pay for that wall. Mark my words." Reuters Donald Trump's most controversial quotes On Free Trade: "Free trade is terrible. Free trade can be wonderful if you have smart people. But we have stupid people." PAUL J. RICHARDS | AFP | Getty Images Donald Trump's most controversial quotes On Mexicans: "When Mexico sends its people, they’re not sending their best. They’re sending people that have lots of problems. They’re bringing drugs. They’re bringing crime. They’re rapists." Getty Donald Trump's most controversial quotes On China: "I just sold an apartment for $15 million to somebody from China. Am I supposed to dislike them?... I love China. The biggest bank in the world is from China. You know where their United States headquarters is located? In this building, in Trump Tower." Getty Images Donald Trump's most controversial quotes On work: "If you're interested in 'balancing' work and pleasure, stop trying to balance them. Instead make your work more pleasurable." AP Donald Trump's most controversial quotes On success: "What separates the winners from the losers is how a person reacts to each new twist of fate." Donald Trump's most controversial quotes On life: "Everything in life is luck." AFP Donald Trump's most controversial quotes On ambition: "You have to think anyway, so why not think big?" Getty Images Donald Trump's most controversial quotes On his opponents: "Bush is totally in favour of Common Core. I don't see how he can possibly get the nomination. He's weak on immigration. He's in favour of Common Core. How the hell can you vote for this guy? You just can't do it." Reuters Donald Trump's most controversial quotes On Obamacare: "You have to be hit by a tractor, literally, a tractor, to use it, because the deductibles are so high. It's virtually useless. And remember the $5 billion web site?... I have so many web sites, I have them all over the place. I hire people, they do a web site. It costs me $3." Getty Images Donald Trump's most controversial quotes On Barack Obama: "Obama is going to be out playing golf. He might be on one of my courses. I would invite him. I have the best courses in the world. I have one right next to the White House." PA Donald Trump's most controversial quotes On himself: "Love him or hate him, Trump is a man who is certain about what he wants and sets out to get it, no holds barred. Women find his power almost as much of a turn-on as his money." Getty Images Donald Trump's most controversial quotes On America: "The American Dream is dead. But if I get elected president I will bring it back bigger and better and stronger than ever before and we will make America great again." GETTY

Yet it may that Ohio, where polls suggest he is running a narrow second to Ohio Governor John Kasich, becomes the most important battleground.

If Mr Kasich wins the state and it’s haul of 66 delegates, it would be impossible for Mr Trump to secure the Republican nomination before the party’s convention in July.

Given the billionaire’s unpopularity among the Republican establishment, it is likely that unless he has a clear majority of delegates going into the convention, he could face a stiff challenge for the nomination from another candidate.

“I'm not running to block anybody, but the math is real,” Mr Kasich told reporters after an event on Monday in Youngstown at Brilex Industry.

A contested convention would be a very contentious one, as Mr Trump’s supporters would revolt against what they see as efforts to steal the nomination.

“It would be a meltdown,” Republican consultant Bruce Haynes told USA Today.

Trump security rushes to stage during rally

Those attending Mr Trump’s rally on Monday evening in Youngstown, a former industrial hub that has for decades been fighting economic collapse, drug problems and depopulation, appeared overwhelmingly of the opinion that Mr Trump alone could help tackle the problems facing the country.

“I think he will do the right thing for America,” said Haskell Westmoreland, 79, a military veteran and former steel worker. “He is going to knock the hell out of Isis. He says he will build a wall, I believe he will.”