Donald Trump is the last man standing in the race for the Republican US presidential nomination as one after the other his sole remaining rivals, Ted Cruz and John Kasich, faded out of the picture.

Anointed the presumptive nominee after winning Indiana this week, the 69-year-old New York billionaire turned to a new phase in his campaign, planning to set up a vice presidential selection committee and stepping up efforts to seek unity with more Republicans.

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 the November 8 elections.

Kasich, the Ohio governor who stayed in the race in hopes of persuading Republicans to choose him as the nominee at a contested convention in July, has confirmed he has quit the race.

Trump has overcome sustained opposition from Republican rivals by using his star power to give voice to an anti-establishment movement.
His supporters are enthusiastic about his America First platform, which has strayed far from typical conservative policy like free trade and military interventionism.

Trump's immediate challenge is to appease Republican loyalists, who are appalled by his bombastic, bullying style, denigrating comments about women and proposals to build a wall on the Mexico border and deport 11 million illegal immigrants.

In a series of interviews, Trump made clear he would not be looking to placate everyone.

"I am confident that I can unite much of it, some of it I don't want," Trump said on NBC's Today show.

Republican National Committee Chairman Reince Priebus tweeted Trump is the party's presumptive nominee, while calling for unity and focus to defeat Hillary Clinton.

Trump believes more Republicans will support him when they consider the possibility of the former secretary of state as president.

Clinton lost the Indiana primary to her challenger, US Senator Bernie Sanders, but remains on course her party's nominee.

She called Trump a "loose cannon" on Wednesday and said America should not take a risk on an unreliable candidate.

The New York Times reports Trump will soon form a team to help him find a vice presidential nominee to be announced in July and has put Ben Carson on the committee.

Carson endorsed Trump after dropping out of the race.

Trump, a former reality television star who has never held public office, suggested he might make a more conventional choice as his running mate.

"I'm more inclined to go with a political person," Trump told the Times.

Support for Trump among Republicans nationally jumped in recent weeks to the highest level of the primary campaign, according to Reuters/Ipsos polling.

A recent poll found Trump with the support of 53 per cent of Republican participants, well above Cruz at 25 per cent and Kasich at 16 per cent.

In a potential general election match-up, however, Clinton led Trump by about 10 percentage points among likely voters.

The poll included 623 Democrats and 556 Republicans and had a credibility interval of five percentage points.