* Democratic socialist holds 47% of county convention delegates
* Six in 10 voters back Medicare for All - Edison Research
LAS VEGAS, Feb 23 (Reuters) - Broad-based support across age, racial and ideological groups propelled Bernie Sanders to a dominant victory in Nevada's Democratic caucuses, tightening his grip on the front-runner spot in the race to find a challenger to President Donald Trump.
Joe Biden, the former vice president, appeared headed to a badly needed second-place finish in Nevada after poor showings earlier this month in the first two nominating contests in the Democratic presidential race ahead of the Nov. 3 election.
Sanders' triumph on Saturday in the first racially diverse state in the campaign suggested he was reaching a broader coalition of Democratic voters with his unapologetic message of social and economic justice, including his signature pledge to provide universal healthcare for all Americans.
For Biden and other moderates who argue Sanders is too liberal to beat Trump and who have been trying to blunt his momentum, however, the Nevada results made the job much harder.
"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," Sanders, a U.S. senator from Vermont and self-described democratic socialist, told cheering supporters in San Antonio, Texas.
With 50% of the precincts reporting, Sanders had 47% of the county convention delegates in Nevada. Biden was a distant second to Sanders with 19%, but ahead of former Mayor Pete Buttigieg of South Bend, Indiana, in third with 15%.
"The press is ready to declare people dead quickly, but were alive and were coming back and were gonna win," Biden told supporters in Las Vegas on Saturday night.
Senator Elizabeth Warren, who had been looking to jump-start her campaign after poor finishes in Iowa and New Hampshire, trailed in a disappointing fourth with 10% in Nevada. Senator Amy Klobuchar and activist billionaire Tom Steyer were well back at 5% and 4%, respectively.
Buttigieg cautioned Democrats about nominating Sanders, portraying him as an ideologue who would divide the country and lose to Trump.
"We can prioritize 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 told supporters in Las Vegas.
The race now begins to broaden across the country, with the next primary on Saturday in South Carolina, followed closely by the Super Tuesday contests in 14 states on March 3 that pick more than one-third of the pledged delegates who will help select a Democratic nominee.
Biden, the No. 2 to former President Barack Obama, is counting on a strong showing in South Carolina, which has a large bloc of black voters. In Nevada, entrance polls showed Biden led among African Americans with 36%, followed by Sanders with 27%.
The Super Tuesday states will be the first nominating contests for former New York Mayor Michael Bloomberg, who has not been competing in the four early voting states but had been rising in opinion polls.
"The Nevada results reinforce the reality that this fragmented field is putting Bernie Sanders on pace to amass an insurmountable delegate lead," Bloomberg campaign manager Kevin Sheekey said in a statement.
Voters poured into more than 250 caucus sites around Nevada, where Sanders was aided by strong support from the six in 10 voters who said they backed a government-run Medicare for All, the Edison entrance poll showed.
The entrance poll showed Sanders led in Nevada across all age groups except for those older than 65. Around 54% of Latino voters said they backed him, while 24% of college-educated white women and 34% of those who have a union member in their families supported him.
He also won with college graduates, and was the top pick of voters who consider themselves independents. He also was favored over Biden among voters whose top priority is defeating Trump in the November election.
Warren shrugged off her poor finish in Nevada, saying she got a boost in fundraising and support from an aggressive debate performance on Wednesday - which came too late to affect early voting in the first part of the week.
"We have a lot of states to go, and right now I can feel the momentum," Warren said at a rally in Seattle.
On Twitter, Trump appeared to be enjoying the Democratic race.
"Looks like Crazy Bernie is doing well in the Great State of Nevada. Biden & the rest look weak, & no way Mini Mike," Trump wrote, the last a reference to Bloomberg.
Nevada caucus officials and voters at multiple sites on Saturday reported voting rules confusion, calculation glitches and delays in reporting tallies - despite efforts to avoid the issues that plagued Iowa's caucuses earlier this month.
After a technical meltdown delayed results in Iowa, state officials promised a revised reporting system using a telephone hotline and photos of caucus reporting sheets would ensure a smoother process. Nevertheless, precinct chairs at some caucuses experienced long waits on the phone lines.
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 Obama's candidacy electrified the party.
But those early votes had to be counted along with those cast on Saturday, slowing the process.
(Reporting by Sharon Bernstein in Las Vegas and John Whitesides in Washington; Additional reporting by Simon Lewis, Doina Chiacu, Ginger Gibson, Jane Ross and Elizabeth Culliford; Writing by John Whitesides; Editing by Howard Goller)