Anointed the presumptive nominee after winning Indiana on Tuesday and driving his closest rival, US Senator Ted Cruz, from the race, the 69-year-old New York billionaire planned to set up a vice presidential selection committee and step up efforts to seek unity among a wider group of Republicans ahead of the Nov. 8 election.

Trump's win in Indiana cleared the way for him to prepare for a likely general election match-up against Democrat Hillary Clinton. The former secretary of state lost the Indiana primary to tenacious challenger US Senator Bernie Sanders, but remains on course to become her party's nominee.

Trump told reporters he would probably work with the Republican National Committee to raise about USD 1 billion for the general election campaign.

Kasich, the Ohio governor, had stayed in the race in hopes of persuading Republicans to choose him as the nominee at a contested convention in July. He ended his campaign as a clear path emerged for Trump to amass the delegates needed to secure the nomination outright.

"As I suspend my campaign today, I have renewed faith, deeper faith that the Lord will show me the way forward and fulfill the purpose of my life," Kasich said in Columbus, the Ohio state capital.

Some Republican leaders said they would support Trump since he would be the nominee, stressing the importance of defeating Clinton in the general election. But there was no mad rush to support him as is typically the case when a presumptive nominee is crowned.

Former President George W Bush, whose brother Jeb was defeated by Trump in the primary campaign, made clear he was staying out of the race. "President Bush does not plan to participate in or comment on the presidential campaign," said his spokesman, Freddy Ford.

A similar statement was issued by the spokesman for Bush's father, former President George HW Bush. "At age 91, President Bush is retired from politics,” spokesman Jim McGrath said.

John McCain, the 2008 Republican nominee, said he would support Trump. McCain is a U.S. senator from Arizona who is seeking re-election this year and was insulted by Trump last year.

"As John McCain has said, he will support the nominee of the Republican Party, who is now presumptively Donald Trump," said McCain's Senate campaign spokeswoman, Lorna Romero.

But the wounds from a brutal primary battle were still raw among many Republican loyalists who simply cannot bear to support Trump because they worry he could spell disaster for the party in November.

Republican Senator Ben Sasse of Nebraska reiterated statements that he would not back Trump and pointed to a February Facebook post in which he said he would look for an alternative candidate if Trump became the nominee.

Nebraska's other US Senator, Republican Deb Fischer, made clear in an interview with Nebraska Radio Network that she would support the party's nominee but was not comfortable with Trump.

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