Donald Trump notched a win in Nevada's Republican caucuses on Tuesday as Marco Rubio bid to elbow out Ted Cruz for second place in an increasingly urgent effort to slam the brakes on the Trump juggernaut.

Trump now has three straight victories — in the West, the South and Northeast — a testament to his broad appeal among the mad-as-hell voters making their voices heard in the 2016 presidential race.

With 68 per cent of precincts reporting, Trump amassed 45 per cent of the votes. Marco Rubio was next at 23.6 per cent, just ahead of Ted Cruz at 22.2 per cent.

"We weren't expected a couple of months ago to win this one, you know that right?" Trump said from Treasure Island casino in Las Vegas to his ecstatic supporters. "Of course, if you listen to the pundits, we weren't expected to win too much and now we're winning, winning, winning the country. And soon the country is going to start winning, winning, winning."

Trump's three straight wins come after finishing second in the leadoff Iowa caucuses. He's in a strong position headed into next week's Super Tuesday contests, where voters in a dozen states will cast ballots in presidential primaries.

"It's going to be an amazing two months," said Trump. "We might not even need the two months, folks, to be honest."

Six in 10 caucus goers said they were angry with the way the government is working, and Trump got about half of those angry voters, according to preliminary results of an entrance poll.

Supporters of Republican U.S. presidential candidate Donald Trump celebrate in Las Vegas as television networks declare him the winner of the Nevada Republican caucuses. (Jim Young/Reuters)

Nevada was a critical test for Rubio and Cruz, the two senators battling to emerge as the clear alternative to the GOP front-runner. Rubio was out to prove he can build on recent momentum, while Cruz was looking for a spark to recover from a particularly rocky stretch in his campaign.

Rubio, already campaigning in Michigan as caucus results rolled in, was projecting confidence that he can consolidate the non-Trump voters who have been splintering among an assortment of GOP candidates, saying, "we have incredible room to grow."

Cruz, a fiery conservative popular among voters on the GOP's right, finished a disappointing third in South Carolina after spending much of the past two weeks denying charges of dishonest campaign tactics and defending his integrity. Another disappointing finish in Nevada would raise new questions about his viability heading into a crucial batch of Super Tuesday states on March 1.

The election calendar suggests that if Trump's rivals don't slow him by mid-March, they may not ever. Trump swept all of South Carolina's 50 delegates, giving him a total of 67 compared to Cruz and Rubio who have 11 and 10, respectively.

There were 30 delegates at stake in Nevada, and a whopping 595 available on Super Tuesday. Trump won at least 12 delegates in Nevada, Rubio and Cruz won at least five apiece, and eight are still left to be allocated.

It takes 1,237 delegates to win the Republican nomination for president.

The billionaire businessman, in his victory speech, dismissed the notion that if more GOP candidates drop out of the race, they'll coalesce around an alternative.

"They keep forgetting that when people drop out, we're going to get a lot of votes," said Trump, who earlier in the night crashed a centre where broadcaster Glenn Beck was speaking on behalf of Cruz.

Nevada delegates are based on voting share

Nevada's caucusing played out in schools, community centres and places of worship across the state — a process that's been chaotic in the past.

Count Tracy Brigida, fed up after her husband was laid off from his mining job, among those caucusing for Trump.

"I want a businessman to run the biggest business in the world," Brigida said as she caucused at a Las Vegas high school.

Jeremy Haight drove straight from his marketing job to caucus for Marco Rubio at the same high school.

"He's the most level-headed. He hasn't said anything stupid or crazy ... which is really what I think the country needs," Haight said.

It was Cruz for Megan Ortega, who declared: "He's consistent, he's bold and he's a class act."

Preliminary results of the entrance poll found that about three in 10 early caucus goers said the quality that mattered most to them in choosing a candidate was that he shares their values, slightly more than the quarter who said they want a candidate who can win in November. About a quarter said they want a candidate who can bring change. About two in 10 want one who "tells it like it is."

Republican presidential candidate, Sen. Marco Rubio, R-Fla., seen speaking Tuesday at a campaign stop in Minneapolis, was looking to elbow out Ted Cruz for second place in Nevada. (Jim Mone/The Associated Press)

Through Tuesday, the Republican candidates and the super PACs supporting them had spent a combined $3.8 million US on television and radio advertisements in Nevada — less than a tenth of the $39.3 million spent ahead of last weekend's South Carolina primary, according to Kantar Media's CMAG data.

That primary reduced a GOP field that included a dozen candidates a month ago to five, with former Florida Gov. Jeb Bush the latest to drop out after a disappointing finish in South Carolina. Ohio Gov. John Kasich and retired neurosurgeon Ben Carson remain in the race and could play spoilers as the trio of leading candidates.

Carson had 5.1 per cent of the Nevada vote, with Kasich at 3.5 per cent.

Trump's rivals concede they are running out of time to stop him.

A suporter of Republican presidential candidate U.S. Sen Ted Cruz (R-TX) looks at her phone during a caucus night gathering at the Bill and Lillie Heinrich YMCA on Tuesday night in Las Vegas. (Justin Sullivan/Getty Images)

After finishing third in Iowa, fifth in New Hampshire and second in South Carolina, Rubio needs a win soon to support the idea that he is the prime heir to Bush's supporters.

Indeed, Republican establishment heavyweights have been flooding to Rubio in recent days, including Arkansas Gov. Asa Hutchinson, North Carolina Sen. Thom Tillis and Utah Sen. Orrin Hatch. South Florida's three Cuban-American members of Congress announced their support for him in the hours before the Nevada contest.

The entrance poll survey was conducted for The Associated Press and the television networks by Edison Research as Republican voters arrived at 25 randomly selected caucus sites in Nevada.