The nationally televised debate will give many voters their first unscripted look at Bloomberg, a media mogul and former New York mayor whose campaign has been fueled by hundreds of millions of dollars of self-funded television ads and carefully choreographed personal appearances.

Despite skipping the first four early voting states in February to focus on later nominating contests in March, Bloomberg qualified on Tuesday for his first debate after meeting the Democratic National Committee's polling requirement.

He will join Bernie Sanders, Joe Biden, Amy Klobuchar, Pete Buttigieg and Elizabeth Warren at Wednesday's debate, three days before Nevada's caucuses, the third contest in the state-by-state race to find a challenger to Republican President Donald Trump in the November 3 election.

Biden and Warren, in particular, face the do-or-die task of reigniting their campaigns after poor showings in Iowa and New Hampshire earlier this month.

Bloomberg, 78, has come under heavy fire on the campaign trail recently as his poll numbers have surged and his entry into the race on March 3 - known as Super Tuesday, when 14 states vote - draws closer.

He has risen to No. 2 among Democrats behind progressive Senator Bernie Sanders, according to a Reuters/Ipsos national poll released on Tuesday.

Rivals are certain to challenge Bloomberg over his record, including his past support in New York of "stop-and-frisk" police policies during his time as mayor that disproportionately hit African Americans.

How Bloomberg, who has not participated in a political debate since his 2009 mayoral re-election campaign, deals with the pressure will help determine his campaign's fate.

"You only have one chance to make a good first impression," said Aaron Kall, director of debate at the University of Michigan. "If he doesn't turn in a good performance, all the momentum he has could evaporate before he is even on a ballot on Super Tuesday."

The Nevada caucuses are the first in a state with a more diverse population after contests in predominantly white Iowa and New Hampshire.

Those first contests produced a split verdict, with Buttigieg, 38, the moderate former mayor of South Bend, Indiana, edging Sanders in Iowa, and Sanders, a senator from Vermont, narrowly beating Buttigieg in New Hampshire.

Bloomberg's presence could be a gift to Sanders, 78, drawing attention and attacks away from the newly minted front-runner, who has surged into the lead in national and Nevada polls after strong finishes in the first two contests.

Warren and Sanders, who regularly rails on the campaign trail against the political influence of billionaires, have accused Bloomberg of trying to buy his way into the White House.

Bloomberg has spent $US338 million of his own money on a barrage of television ads, according to independent ad tracker Advertising Analytics.

Bloomberg, seemingly eager to turn the race into a one-on-one contest with Sanders, has questioned whether Sanders and his fervent online supporters - known as "Bernie Bros" - were hurting the party.

Bloomberg will not be the only candidate under pressure.

Biden, 77, the former vice president and onetime front-runner, and Warren, 70, a senator from Massachusetts who was considered a leading contender just three months ago, will be fighting for their political survival in Nevada.

Klobuchar, 59, who surged into a third-place finish in New Hampshire after a strong debate performance there, and Buttigieg will face questions about what polls show is their lack of support among black and Latino voters.

Nearly one-third of voters in the Nevada Democratic caucuses in 2016 were either black or Latino, according to entrance polls.

Bloomberg qualified for the debate after the Democratic National Committee eliminated requirements on the number of donors that had previously blocked the self-funding Bloomberg.

Australian Associated Press