In a stunning triumph for a political outsider, Donald Trump all but clinched the Republican presidential nomination Tuesday with a resounding victory in Indiana that knocked rival Ted Cruz out of the race and cleared Trump's path to a likely November face-off with Democratic front-runner Hillary Clinton.

Trump still needs about 200 delegates to formally secure the nomination, but Cruz's decision to end his campaign removed his last major obstacle.

The conservative firebrand and Texas senator, who had no chance of clinching the nomination before the convention, pinned his presidential hopes on Indiana and tried to cast himself as the only viable alternative to the billionaire businessman.

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"I've said I would continue on as long as there was a viable path to victory; tonight I'm sorry to say it appears that path has been foreclosed," Cruz told a sombre crowd in Indianapolis.

Cruz spent the past week in Indiana, securing the support of the state's governor and announcing Carly Fiorina, a retired tech executive and former presidential hopeful, as his running mate. He also guaranteed the nomination would head to a convention contest while making the rounds on the Sunday morning talk shows two days ago.

He had also formed a so-called pact with Kasich so that the Ohio governor would not campaign in Indiana, giving Cruz a better chance at winning, though some have said that move was ineffective and a mistake.

Cruz drops out0:38

Trump also devoted much of his attention to Indiana, spending more time there than he has in most other states, underscoring his eagerness to put Cruz behind him and turn his attention toward Democratic front-runner Hillary Clinton and a general election.

"Ted Cruz — I don't know if he likes me or he doesn't like me — but he is one hell of a competitor," Trump said of his last fierce competitor whom he had dubbed "lyin' Ted." Trump, in a victory speech that was much lower-key than usual, promised victory in November, vowing anew to put "America first."

One outside group trying to stop Trump suggested it would shift its attention to helping Republicans in other races. Rory Cooper, a senior adviser to the Never Trump super PAC, said the group will help protect "Republican incumbents and down-ballot candidates, by distinguishing their values and principles from that of Trump, and protecting them from a wave election."

Trump now has at least 1,047 delegates. Cruz exits the race with 565, while Kasich has 152.

Kasich pledged to stay in the race, with his campaign manager saying the governor would continue to "offer the voters a clear choice for our country."

Sanders wins Democratic Indiana primary

Vermont Senator Bernie Sanders eked out a victory over Clinton in Indiana, but the outcome will not slow the former secretary of state's march to the Democratic nomination. Heading into Tuesday's voting, Clinton had 92 per cent of the delegates she needs.

"I know that the Clinton campaign thinks this campaign is over. They're wrong," Sanders said defiantly in an interview Tuesday night.

But Clinton already has turned her attention to the general election.

She and Trump now plunge into a six-month battle for the presidency, with the future of America's immigration laws, health care system and military posture around the world at stake. While Clinton heads into the general election with significant advantages with minority voters and women, Democrats have vowed to not underestimate Trump as his Republican rivals did for too long.

Republican National Committee chairman Reince Priebus declared the race over, saying on Twitter that Trump would be the party's presumptive nominee.

"We all need to unite and focus on defeating (at)HillaryClinton," he wrote.

Though Sanders claimed momentum, he has conceded his strategy hinges on persuading superdelegates to back him over the former secretary of state. Superdelegates are Democratic Party insiders who can support the candidate of their choice, regardless of how their states vote. And they favor Clinton by a nearly 18-1 margin.

Exit polls showed about seven in 10 Indiana Democrats said they'd be excited or at least optimistic about either a Clinton or Sanders presidency. Most said they would support either in November.

The exit polls were conducted by Edison Research for The Associated Press and television networks.

With Sanders's narrow victory Tuesday, he picked up at least 43 of Indiana's 83 delegates. Clinton now has 2,202 delegates to Sanders' 1,400. That includes pledged delegates from primaries and caucuses, as well as superdelegates.