All eyes will be on the US media magnate as he navigates a 2020 national audience for the first time, after his astronomical spending on campaign advertising fueled a rise in polling that has sent him into the top tier of candidates.

Bernie Sanders, meanwhile, will take the stage as the clear frontrunner, buoyed by a strong showing in Iowa, a victory in New Hampshire and a surge in polling with the next nominating contest just three days away in Nevada.

A Washington Post-ABC News poll out Wednesday showed Sanders with a commanding double digit lead nationally, at 32 percent. 

Sagging former frontrunner Joe Biden, who has suffered the most from Bloomberg's gains, was second at 16 percent, followed by Bloomberg at 14 and Elizabeth Warren at 12.  

While Sanders and other White House hopefuls have spent months barnstorming early states, billionaire Bloomberg parachuted late into the Democratic contest.

His surge boosts the chances of November's election being a unique show of one septuagenarian white male New York billionaire challenging another -- Trump himself.

The prospect is not sitting well with Bloomberg's five debate rivals, who are likely to hurl searing salvos at him when they take the stage for a two-hour Las Vegas showdown starting 6:00 pm local time (0200 GMT Thursday).

Sanders and others have already hammered Bloomberg hard, stressing Americans have no appetite for billionaires trying to "buy" their way into the presidential race.

Bloomberg has essentially foregone the campaigning in the first four contests, including Nevada.

Instead he is going all in on so-called Super Tuesday on March 3, when 14 states including California and Texas vote on choosing a Democratic nominee.

The ninth Democratic debate's other participants -- former vice president Biden, former South Bend, Indiana mayor Pete Buttigieg and Senator Amy Klobuchar, all moderates, and the more progressive senators Sanders and Warren -- have castigated Bloomberg for his approach.

 'Egomaniac billionaire' 

Buttigieg, asked by CNN if he thought Bloomberg was trying to buy his way in, was unequivocal.

"What else do you call it when you dip into your endless reserves of millions and billions and don't go through the process of campaigning in states like Nevada, or Iowa, or New Hampshire, humbling yourself... looking eye to eye to voters?" Buttigieg responded.

Biden attacked the political affiliations of Bloomberg, a longtime Democrat who left the party to run for mayor first as a Republican, then an independent. In 2018 he re-registered as a Democrat.

"He basically has been a Republican his whole life," Biden, 77, fumed Wednesday.

Sanders and Bloomberg, both 78, have also tangled in increasingly ugly ways.

On Wednesday, Bloomberg spokesman Tim O'Brien accused the Sanders campaign of behaving in "Trumpy" fashion by falsely asserting that Bloomberg has had a heart attack. 

He did have stents installed in 2000 because of a coronary blockage but has never had a cardiac arrest. Sanders had a heart attack in October.

"Those are the facts," O'Brien tweeted. "It's a dangerous time when Sanders goes all in with Trumpism."

While Sanders is ahead, Bloomberg is surging on the national stage. Two separate polls released Tuesday show him leapfrogging rivals to claim second spot behind Sanders, with Biden third.

Buttigieg, who narrowly won Iowa and finished second in New Hampshire, will seek to show viewers that his strong early performances were no fluke.

 Sharp criticism 

For Warren, Klobuchar and Biden, the Nevada debate is a critical opportunity for them to convince voters that they belong in the race heading into the stretch.

Bloomberg has his own challenge: the powerful billionaire who has long controlled his message will have to defend his record in real time against practiced politicians eager to notch a viral moment.

Some of Bloomberg's policies as mayor are facing sharp criticism.

Several rivals have highlighted the stop-and-frisk police operations during Bloomberg's mayorship that disproportionately targeted people of color.

Bloomberg has apologized for enforcing the policy, and on Saturday acknowledged that he defended it "for too long" because he failed to understand the "unintended pain" it caused minorities.

On the streets of Las Vegas, where protesters fighting for a union contract gathered Wednesday, some voters were rejecting a candidate who has opted to skip Nevada and other early-voting states.

"He's trying to buy votes," said pastor Sylvester Rogers, 79. 

Bloomberg "should be in the race from the beginning, like every other candidate," Rogers added. "He needs to see what our communities are about."

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