LAS VEGAS — Ted Cruz is fighting to keep from spiraling out of contention.
Marco Rubio is fighting to prove he can build on his recent momentum.
And Donald Trump, with his rivals locked in a battle for second, is fighting for a third straight victory to expand a delegate lead that could soon become insurmountable.
Tuesday night's Nevada caucuses force a critical test on the Republican Party's leading presidential candidates.
"I think it's the most unpredictable of all the races we've had so far. You go in really knowing less about this than any other election," said a less-than-confident Rubio. "We'll see."
Cruz, a fiery conservative popular among voters on the GOP's right, needs a spark to recover from one of the weakest moments of his campaign.
After denying charges of dishonest tactics for several weeks, the Texas senator on Monday asked for the resignation of a senior aide who spread an inaccurate news report suggesting Rubio had criticized the Bible. That was just days after Cruz finished a disappointing third in South Carolina after spending much of the past two weeks defending his integrity.
Another disappointing finish in Nevada's low-turnout caucuses would raise new questions about Cruz's viability heading into a crucial batch of Super Tuesday states on March 1.
"There's something wrong with this guy," Trump said with his usual measure of tact during a massive Las Vegas rally Monday night. The former reality television star called the Texas senator "sick."
Nevada voting takes place in schools, community centers and places of worship across the state. Unlike a primary, which involves quick and solitary voting, caucuses involve a more public pronouncement of presidential preference including speeches to try to persuade undecided neighbors.
The state marks the first Republican election in the West, the fourth of the campaign.
A GOP field that included a dozen candidates a month ago has been reduced 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, Trump, Cruz and Rubio, battle for delegates with an increasing sense of urgency.
Trump's rivals concede they are running out of time to stop him.
The election calendar suggests that if the New York billionaire'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 are 30 delegates at stake in Nevada, awarded to candidates in proportion to their share of the statewide vote so long as they earn at least 3.33 percent. While proportional contests give Trump's weaker rivals a chance to accumulate delegates, proportional contests also make it difficult to catch up if one candidate runs up a significant lead.
Rubio and Cruz have been laying into each other viciously in recent days, an indication they know Trump can be stopped only if one of them is eliminated. But neither of the first-term Hispanic senators is predicting victory in Nevada.
Rubio left the state before voting began, preferring to campaign in Minnesota and Michigan instead. He lived here as a boy, from grades 3 through 8, and Nevada-based family members and friends poured into his Las Vegas-area rallies in recent days as he shared stories about his formative years.
After finishes of third in Iowa, fifth in New Hampshire and second in South Carolina, Rubio needs a win soon to support his theory that he is the primary beneficiary of Bush's departure.
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.
"We have incredible room to grow," Rubio told reporters during a Monday night news conference on his campaign plane.
After accusing Cruz of leading a campaign culture of "lies," the Florida senator asserted that Trump's support is capped at roughly one-third of the Republican electorate.
"That means 65 percent of the party is against him," Rubio said. "The problem is that 65 percent has been divided up among a lot of people. As long as that continues, he'll succeed. But once it consolidates against an alternative he'll lose."
Nancy Benac reported from Washington.