Trump the sole presidential Republican candidate left

Now that he is the sole presidential candidate of the Republican Party, Trump's immediate challenge is to unite deep fractures within the party, as many party loyalists are appalled at his bullying style, his treatment of women and his signature proposals to build a wall on the border with Mexico and deport 11 million illegal immigrants.

The former reality TV star himself called for unity in a speech at a victory rally that was free of his usual bombast and flamboyance. Calling Indiana a "tremendous victory", he immediately directed fire at Democratic frontrunner Hillary Clinton.

"We're going after Hillary Clinton," he said. "She will not be a great president, she will not be a good president, she will be a poor president. She doesn't understand trade."

On the same night, Clinton took a low in Indiana as her rival Bernie Sanders unexpectedly tore victory out of her hands. Sanders, a self-declared socialist, beat Clinton by 53.2 percent to 46.8 percent with about three quarters of precincts reporting - although Clinton remained well ahead in the overall delegate battle for the nomination.

"Bernie Sanders was behind several points just a few weeks ago. Thousands were turning up to his rallies even in thunderstorms to hear what he had to say," Al Jazeera's, reporting from Indianapolis, said.

"A narrow victory in Indiana is enough to re-inject his campaign with momentum and for him to say that he is going to take it all the way to Democratic convention in Philadelphia in the summer," said Al Jazeera reporter Alan Fisher.

As the race was called overwhelmingly in Trump's favour, Cruz conceded to supporters in Indianapolis that he no longer had a viable path forwards.

"We left it all on the field in Indiana," Cruz said. "We gave it everything we've got, but the voters chose another path. And so with a heavy heart, but with boundless optimism for the long-term future of our nation, we are suspending our campaign."