Kasich quits Republican race to leave Trump last man standing

Billionaire Donald Trump's last remaining Republican challenger, Ohio Governor John Kasich, dropped out of the US presidential race on Wednesday, making Trump officially the party's presumptive nominee.

"As I suspend my campaign today, I have renewed faith - deeper faith - that the Lord will show me the way forward," the low-polling Kasich told supporters in Ohio.

Trump meanwhile pledged to unify the fractured Republican Party after a bruising primary season, as he declared the battle for the White House starts now against likely Democratic nominee Hillary Clinton.

The real estate mogul's path to the GOP presidential nomination is clear after his commanding victory in Indiana's primary on Tuesday also pushed Texas Senator Ted Cruz out of the race.

"Now we'll unify the party. We're going to get people together," Trump, 69, told Fox News. "I think we'll beat Hillary Clinton."

But with unfavourability ratings among the highest of any modern presidential candidate, and concern within his own party about his temperament, Trump swiftly sought to assuage concerns about how he would govern.

"I know people aren't sure right now what a President Trump will be like," he told the New York Times.

"But things will be fine. I'm not running for president to make things unstable for the country."

'They loved me'

Trump also began discussing the idea of his possible running mate, telling ABC News he wanted "a person with political experience" to compliment his own business acumen.

Names being tossed around included Senator Rob Portman of critical swing state Ohio, and South Carolina Governor Nikki Haley, who as the daughter of Indian immigrants could strengthen Trump's appeal with minorities.

Trump himself said he could consider Kasich, a former congressman who helped balance the federal budget, as his vice presidential pick.

Kasich rarely followed rivals like Trump into the mud, aiming to stay above the fray and offer Americans optimism instead of insults and negativity.

In bowing out Wednesday, he grew wistful about the thousands of Americans he met on the campaign trail.

"They loved me, they encouraged me. The people of our country changed me... with stories of their lives," he said.

Just as Trump seized the GOP mantle, a new CNN poll emerged that highlighted the formidable challenge facing him.

It found Clinton leading the tycoon 54 percent to 41 percent - her largest lead since July.

The former secretary of state, however, suffered a shock loss in Indiana to her challenger Bernie Sanders, who has pledged to remain in the race until the end in spite of an extremely steep hill to climb.

Sanders has consistently polled even better against Trump than has Clinton.