Donald Trump, the US Republican Party’s presumptive presidential nominee, has dismissed unity of the party as a prerequisite for winning the White House.

“I think it would be better if it were unified,” Trump said in an interview with NBC News on Sunday. “And I think there would be something good about it. But I don’t think it actually has to be.”

Trump made the remarks after several Republican politicians, including Paul Ryan, John McCain, Lindsey Graham, Mitt Romney, Jeb Bush, George W. Bush, and George H. W. Bush, refused to endorse his candidacy.

The billionaire businessman, who has never held elected public office, has called the Republican Party a “rigged” party, and accused senior GOP politicians of plotting against him.

“The bosses are trying to run it. It’s a rigged party. The bosses want to pick whoever they want to pick. The voters wouldn’t stand for it,” Trump told CNN last week.

However, after winning in Indiana last week, Trump promised to unite both the Republican Party and the country.

“This country, which is divided in so many ways, is going to become one beautiful, loving country,” he said.

In a series of interviews on Sunday, Trump said he was surprised last week when US House Speaker Ryan declined to offer support. The businessman told NBC News that after his victory in the New York primary two weeks earlier, Ryan called to congratulate him.

“Then all of a sudden he gets on and he does this number,” he said. “So I’m not exactly sure what he has in mind.”

On Thursday, Ryan told CNN he is "just not ready" to endorse the presumptive Republican nominee, hours after US Senate Majority Leader Mitch McConnell said he is committed to supporting Trump's presidential bid and called on fellow Republicans to do the same.

After Trump's commanding victory in Indiana's primary last week, his remaining challengers, Ted Cruz and John Kasich, both suspended their presidential bids, leaving the businessman tycoon on an uncontested path to the nomination.

Trump's campaign has been marred by his defamatory remarks against minorities in the US. His comments include calling for a total ban on Muslims from coming to America and forced deportation of Mexican migrants.

The race for Republican nomination saw countless instances of scandals, mudslinging and onstage vulgarity, where rivals time and again seized opportunities to blacken their opponents and push them down the gutter.