Bernie Sanders claimed a decisive victory in the Democratic presidential caucuses in Nevada on Saturday, while Joe Biden appeared headed to a second-place finish that would give his struggling campaign new hope.

Sanders, a senator from Vermont, rode a wave of backing from a diverse coalition of young and middle-aged voters, Latinos, union members and white college-educated women to a win in Nevada, showing signs of expanding support for his front-running campaign beyond his longstanding core.

"We won the popular vote in Iowa, we won the New Hampshire primary, and according to three networks and the AP, we have now won the Nevada caucus," the 78-year-old told cheering supporters in San Antonio, Texas.

"We have put together a multi-generational, multiracial coalition that is going to not only win in Nevada, it's going to sweep the country."

Biden, the former vice president, had been in desperate need of a strong showing after poor finishes in the first two contests in Iowa and New Hampshire for the party's nomination to face Republican President Donald Trump in the November election.

He was a distant second to Sanders with 19 percent of the vote with 10 percent of the precincts reported but ahead of former Mayor Pete Buttigieg of South Bend, Indiana, in third with 16 percent.

"The press is ready to declare people dead quickly, but we're alive and we're coming back and we're gonna win," Biden told supporters in Las Vegas.

Buttigieg congratulated Sanders, but then told his supporters in Las Vegas that Democrats should stop and reconsider nominating Sanders, a self-identified democratic socialist that he portrayed as an ideologue.

"We can prioritise either ideological purity or inclusive victory. We can either call people names online or we can call them into our movement. We can either tighten a narrow and hardcore base or open the tent to a new, broad, big-hearted American coalition," Buttigieg said.

New York billionaire Mike Bloomberg, who dominated the political conversation this week after a poor debate-stage debut, was not on the ballot. He is betting everything on a series of delegate-rich states that begin voting next month.

Supporters cheer as they watch returns from the Nevada caucuses while waiting for Bernie Sanders to arrive for a campaign rally at Cowboys Dancehall on February 22, 2020, in San Antonio, Texas [Drew Angerer/Getty Images/AFP]

Nevada was the third contest on a 2020 election calendar marked by chaos and uncertainty after the opening votes in Iowa and New Hampshire, overwhelmingly white, rural states.

The first presidential contest in the West tested the candidates' strength with black and Latino voters for the first time in 2020. Nevada's population aligns more with the US as a whole, compared with Iowa and New Hampshire: 29 percent Latino, 10 percent black and 9 percent Asian American and Pacific Islander.

The Nevada verdict represented the third in a primary season that will span all 50 states and several US territories, ending only at the party's national convention in July. But with two more rounds of voting scheduled over the next 10 days - including Super Tuesday's massive delegate haul - the party may identify a consensus candidate long before the convention.

Steve Clemons, host of Al Jazeera's weekly show The Bottom Line, called the margin of victory for Sanders "unexpected".

" I think the [Democratic race] is going to winnow out more quickly than people thought, given Bernie Sanders' strength," he said from Chestertown in Maryland. "Joe Biden is acting confidently, but at 19 percent in a state like Nevada, many people are looking at that fairly shocked. Elizabeth Warren has done very poorly and Amy Klobuchar barely registered in this race and this has got to be a concern for her."

If Sanders wins the Democratic nomination, the November election was likely to be "intense", he said.

"We are going to see an incredibly passionate turnout because what's clear about Bernie Sanders, where he is in the Democratic Party and the contest he's giving to the so-called 'moderates' is that the intensity of [his supporters'] passion for him, is about as great as the intensity of passion that Donald Trump's support base feel for the president.

"If this comes to be, and Bernie Sanders wins the Democratic primary, it will be an incredibly intense election."

Trump offered a congratulation of sorts to Sanders.

"Looks like Crazy Bernie is doing well in the Great State of Nevada," Trump tweeted, maligning other candidates before adding: "Congratulations Bernie, & don't let them take it away from you!"

The stakes were high for Nevada Democrats to avoid a repeat of the chaos in Iowa, and officials promised that a revised reporting system using a telephone hotline and photos of caucus reporting sheets would ensure a smoother process.

But precinct chairs at some caucuses reported long waits on the phone lines. Larry Van, a retired pharmacist who was the volunteer secretary at a precinct that went to Biden, said he called the phone number to report results eight times before he eventually got through.

In the final result of a caucus at the famed Bellagio hotel on the Las Vegas strip, Sanders finished with 76 votes, Biden had 45 and no other candidate ended with a vote.

Workers at the hotel, who are members of the Culinary Workers Union, streamed out of the caucus after backing Sanders despite their leadership expressing reservations about his healthcare plan.

"I went for Bernie. I'm not big into politics, but I like the things he's going for: student loan debt, schools, free healthcare," said Aleiza Smith, 22, a housekeeper at the Bellagio.

Four days of early voting in Nevada this week drew more than 75,000 Democrats, more than half first-time voters, putting the party in position to surpass the turnout record of 118,000 in 2008, when Barack Obama's candidacy electrified the party.

But those early votes had to be counted along with those cast on Saturday, complicating the process.

An entrance poll by the Edison Research agency showed six in 10 Nevada voters at the caucuses backed the Medicare for All proposal, a version of which is also supported by Warren. Six out of 10 caucus-goers wanted someone who can beat Trump more than someone who agrees with them on major issues, according to the poll.