LAS VEGAS – The fireworks surrounding Wednesday's Democratic presidential debate in Nevada started hours before the prime-time showdown – as Mike Bloomberg clashed with primary rival Joe Biden in a round of political smack talk that offered a preview of what's to come.

The billionaire former New York mayor is the center of attention Wednesday night, as he joins the debate stage for the first time -- thanks to an abrupt party rule change in the qualification criteria. He now faces accusations from virtually the entire field that he's buying his way into contention with unprecedented spending on ads, staff and more.

But it was former Vice President Joe Biden – hemorrhaging in the polls after lackluster showings in the Iowa caucuses and New Hampshire primary – who took some of the toughest shots at the Republican turned independent turned Democrat.

BIDEN'S FIREWALL CRACKING AS BLACK SUPPORT FOR FORMER VP CRUMBLES

“I tell you what. I’m just gonna tell the truth. That always works,” Biden told reporters, after joining picketing Culinary Union workers outside the Palms Casino and Resort in Las Vegas.

“The truth is he’s basically been a Republican his whole life. The fact of the matter is he didn’t endorse Barack or me when we ran. This is a guy using Barack’s pictures like, you know, they’re good buddies. I’m going to talk about his record,” Biden stressed.

Biden’s emphasized his close relationship serving for eight years as then-President Obama’s vice president. Obama – who’s stayed neutral in the 2020 Democratic nomination race – remains extremely popular with Democratic primary voters.

BLOOMBERG'S POLL NUMBERS SOAR WITH AD SPENDING SURGE

While Biden was talking with reporters, a top Biden campaign adviser emphasized that voters would see sharp contrasts on the debate stage, saying, “Mayor Bloomberg is profoundly unvetted.”

And the aide spotlighted Bloomberg’s “flashing red lights” of vulnerability.

Biden's presidential campaign also unveiled a new digital ad mocking Bloomberg’s recent touting of his relationship with Obama. The spot shows clips of Bloomberg criticizing the former president for his signature domestic achievement – ObamaCare – and for implementing financial regulations in the wake of the last decade’s great recession.

The ad includes a clip of Bloomberg slamming the former president’s record on race relations, asking, “Why during the Obama administration didn’t we pull together.” The spot highlights that Bloomberg didn’t endorse Obama in the 2008 election and features a clip of Bloomberg calling President Trump a ”friend” and a “New York icon.”

Bloomberg’s campaign quickly fired back – releasing a new digital spot that shines a spotlight on Biden’s past comments praising the former New York City mayor:

“I don’t know anybody I’ve worked with in my career ... who does more to create the future than you Mike.”

As Biden’s poll numbers have slid following his fourth- and fifth-place finishes in Iowa and New Hampshire, Bloomberg’s seen his poll numbers soar, thanks in part to roughly $400 million he’s spent to run ads across the country on TV, Facebook and Google.

But Wednesday night, Bloomberg will face off for the first time against Biden and his other rivals for the Democratic nomination.

Because he jumped into the race less than three months ago, Bloomberg decided to skip campaigning in Iowa, New Hampshire and the two other early-voting February states: Nevada and South Carolina. Instead, he’s concentrating on the delegate-rich Super Tuesday states that hold contests in two weeks, on March 3, and beyond.

Until the Nevada debate, Bloomberg had no avenue to qualify for the showdowns since one of the criteria was meeting individual donor thresholds. The multibillionaire is self-financing his campaign and not seeking campaign contributions from individuals. That rule was later changed.

Many of his rivals have repeatedly slammed him the past couple of weeks, charging that he’s bought his way onto the debate stage.

"It’s a shame Mike Bloomberg can buy his way into the debate," Sen. Elizabeth Warren of Massachusetts tweeted on Wednesday. "But at least now primary voters curious about how each candidate will take on Donald Trump can get a live demonstration of how we each take on an egomaniac billionaire."

One of the front-runners – populist Sen. Bernie Sanders over Vermont – once again jabbed at Bloomberg on the eve of the debate.

“We’re gonna win this election not because we are buying the airwaves as Mr. Bloomberg is, we’re gonna win this election because we are putting together the strongest grassroots movement that this country has ever seen,” Sanders told supporters in Reno.

At a town hall in Las Vegas on Tuesday night, former South Bend, Ind., Mayor Pete Buttigieg jabbed at Bloomberg as a “billionaire who thinks that you can just buy your way onto TV.”

But, Bloomberg’s free to spend as much money as he’d like on his White House bid. There currently have been no limits or restrictions on how much of his or her own money a federal candidate could spend on a campaign for office.

Piling on Sanders

Besides targeting Bloomberg, the Biden campaign was also firing away at Sanders ahead of the debate.

Spotlighting gun safety reform – an important issue in Nevada in the wake the Las Vegas Strip massacre – a top Biden campaign aide told reporters that “Sanders has put the interest of the NRA (National Rifle Association) and gun manufacturers ahead of the lives of Americans again and again and again.”

Biden advisers also slammed Sanders on his government-run "Medicare-for-All" health care plan, and for past votes on immigration reform.

Meanwhile, Bloomberg’s campaign warned in an internal memo that Sanders will be nearly unstoppable on his way to the Democratic presidential nomination unless other moderate candidates drop out before Super Tuesday.

"As the race stands today, Sanders is poised to leave Super Tuesday with an over-400 delegate lead verses his next closes competitor [Bloomberg] - a likely insurmountable advantage," said the first bullet point of the Feb. 17 memo, first reported by Axios and confirmed by Fox News.

The memo says that if Sanders continues on the path he is on without certain non-Bloomberg candidates dropping out, "It will be all but impossible to stop him from getting to a plurality (or even majority) of pledged delegates."

The document goes on to point out the alleged weaknesses in each of Bloomberg's moderate competitors' campaigns before concluding: "If [Joe] Biden, [Pete] Buttigieg, and [Amy] Klobuchar remain in the race despite having no path to appreciably collecting delegates on Super Tuesday (and beyond), they will propel Sanders to a seemingly insurmountable delegate lead by siphoning votes away from [Bloomberg]."

On the picket lines

Culinary Local 226 – which represents some 60,000 hospitality workers at the casinos and resorts in Las Vegas and Reno – is Nevada’s largest and most politically influential union.

That was more evident than ever on Wednesday when five of the Democratic candidates joined striking union workers picketing outside the Palms Casino and Resort.

Waking the picket lines – in order of appearance – were Warren, Buttigieg, Biden, environmental and progressive advocate Tom Steyer and Sen. Amy Klobuchar of Minnesota.

Last week the union decided to not make an endorsement in the Democratic race – which was considered a blow to Biden – who’s made a hard sell backing organized labor.

Steyer – who failed to qualify for Wednesday night’s debate – took aim at fellow billionaire Bloomberg for not showing up to march in solidarity with the striking workers.

“I don’t see him out here today. I can tell you that,” Steyer told reporters. “He should be here today.”

Steyer – by the way – announced that he’ll spend at least $1 million to run a new TV ad next week in Super Tuesday states that slams Bloomberg over the controversial use of stop and frisk and other discriminatory policing practices during his tenure as New York City mayor.

Early vote surge

The Nevada Democratic Party announced that more than 70,000 people took part in the four days of early caucus voting from Saturday through Tuesday. That’s nearly as many people who voted in the 2016 Democratic caucuses, when there was no early voting option. The state party said on Wednesday that they were still counting the number of early voting ballots and that the number could rise.

In 2008, 118,000 people took part in the Democratic caucuses.

Fox News' Tara Prindiville, Madeleine Rivera, Andres del Aguila, Kelly Phares, Andrew Craft, Alexandra Rego, Lee Ross, and Tyler Olson contributed to this report.