Las Vegas (AFP) - US Republican presidential candidates faced off in Nevada with Donald Trump poised for a third straight victory ahead of next week's all-important "Super Tuesday" votes.
As the caucuses got underway, the billionaire real estate mogul was all but certain to triumph, with the big question being which candidate would come in second.
Voting got underway in the early evening, with initial results expected after the polls close at 9:00 pm (0500 GMT).
The contest is the fourth for the Republican presidential candidates, with Trump so far winning in New Hampshire and South Carolina. He came in second in Iowa.
Although the caucus in Nevada is not expected to have a significant impact on the overall race -- only 30 delegates or slightly more than one percent of the total are up for grabs -- it is the first contest for the Republicans in the US West.
It is also the first test of Republican voter sentiment after Jeb Bush pulled out of the race last week following a poor showing in South Carolina.
And candidates here are facing the most diverse electorate thus far in the race, more representative of the US population as a whole.
Experts said the focus was on whether Florida Senator Marco Rubio and Texas Senator Ted Cruz would be able to slow Trump's momentum and which of the two candidates would come in second.
"Who is going to be the strongest guy to go against Trump... is going to be more clear after today," said Dan Lee, assistant professor of political science at the University of Nevada, Las Vegas.
"Today we are going to see whether voters are going to move over more in support of Rubio."
He noted that the Nevada caucus was taking place as mainstream Republicans are grudgingly accepting the fact that the bombastic Trump may well end up the party's nominee given his seemingly unstoppable winning streak.
"A lot of Republicans -- especially the Republican establishment, professionals, governors -- don't really want Trump to win the nomination," Lee said.
"They want to get Cruz out and have Rubio go against Trump," he added.
"A lot of people think it's the best chance to beat Trump."
- 'Conservative of convenience' -
Katie Packer Gage, who heads Our Principles, a political action committee aimed at derailing Trump, said in a memo that "it's critical we come together to prevent Donald Trump from becoming the GOP nominee."
"He is, to put it simply, a conservative of convenience -- as his own words and actions over the past 30 years have made abundantly clear," she wrote.
The real estate magnate dished out his trademark rhetoric against his rivals ahead of the vote Tuesday, comparing Cruz to a "soft, weak, little baby" at a rally.
"But for lying, he's the best I've ever seen," he added.
Cruz fired back, accusing Trump of consistently vacillating on issues and saying his insults showed how rattled he was.
"@realDonaldTrump, showing class & grace, calls me a 'soft weak little baby,'" Cruz tweeted. "Hope he doesn't try to eat me!"
Rubio, who has received a flood of endorsements from fellow lawmakers who see him as the mainstream candidate who can topple Trump, for his part urged registered Republicans in Nevada to turn out in force for the caucus.
The Republican field, which once stood at 17, has shrunk to five, with Bush the latest to pull out on Saturday.
Apart from Trump, Cruz and Rubio, the remaining two candidates in the race are retired neurosurgeon Ben Carson and Ohio Governor John Kasich.
After Nevada, the real test on where the presidential candidates stand will come on March 1, when 12 states go to the polls in what is known as "Super Tuesday."
Unlike primaries, caucuses allow participants to openly engage with one another and hear arguments from candidates' supporters or surrogates, in meetings at schools, community centers and churches.
Republicans then vote by secret ballot, in 130 caucus sites across Nevada.
The results will be used to determine the number of Republican delegates who represent the state at the party's nominating convention in July.