Donald Trump is back on his home turf ahead of the primary election in New York, and has won a high-profile endorsement from the city's former mayor Rudy Giuliani as he seeks a convincing victory to stabilise his teetering campaign.

Mr Giuliani announced on Thursday night that he would cast his ballot for Mr Trump, describing him as a "negotiator" who would get things done.

The billionaire said he was honoured by the endorsement from the man who became known as "America's Mayor" for his handling of the September 11 attacks in the city.

"Everyone has tremendous respect for Rudy Giuliani and this is such a great honour," he said. "Rudy knows me well and therefore it is an even greater honour than it would normally be."