"I am honoured to be the presumptive nominee for the Republican Party. It is time to unite our party and defeat Hillary Clinton," Trump yesterday said in a message to his supporters after winning the Indiana primary, where he got more than 52 per cent of the votes polled.
His main rival, Senator Ted Cruz trailed behind by more than 16 points.
Cruz later announced to drop out of the Republican presidential race.
"We are going to make America great again," a confident Trump told supporters at his campaign headquarters in New York as he announced to take on Clinton, his potential Democratic rival in the November presidential elections.
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"We are going after Hillary Clinton. She will not be a great president. She will not be a good president. She would be a poor president. She does not understand trade," Trump said, giving a brief glimpse of his campaign against the Democratic presidential nominee.
In his remarks, Trump described Cruz as a tough competitor and appreciated his decision to withdraw from the race.
"Ted Cruz is one hell of a competitor. He is an amazing guy. He has got an amazing future. He is one tough competitor," Trump said.
In his speech that lasted for a little less than 20 minutes, Trump talked about uniting the party and focusing on trade and economic issues.
"We are going to bring back our jobs," Trump said as he warned the US companies with "consequences" if they moved out of the country.
party's nominee at that time, no one took Trump seriously as they did not expect him to stay in the race to the White House for too long.
However, he has not only remained the frontrunner from the first month itself, but since early this year when the primaries began, Trump has shattered the White House dreams of all of them, many of them being Governors and Senators who have been popular and established politician for decades.
Governor John Kasich from Ohio, who has 153 delegates as against 1,047 delegates of Trump, still poses challenge to Trump's nomination as GOP candidate. However, Kasich himself has no path to nomination.